sql: add maker for sqlfluff

[sethwoodworth]

I'm hoping to add a maker for an alternate sql linter named sqlfluff.

I'm new to vim's errorformat and got a partially working &efm for sqlfluff's default output format.

@blueyed seems really responsive and helpful in PRs, so I am hoping to get some feedback and get this PR working and tested!

Questions:

1. sqlfluff takes a --dialect argument. Is there an example maker I could reference for providing a config option for this?
2. sqlfluff has three output format options: human, json, & yaml, my &efm is attempting to match the human output format as I wanted to avoid multi-line errorformats, and json doesn't have line breaks. But after getting message-continuations working, I suspect yaml would be easier than the human format. Suggestions? I can provide example output.
3. sqlfluff has error codes of PRS and L003 the efm of %t%n breaks when the error code is PRS. How should I handle this case?

Here is an example snippet of the human format output:

== [models/redacted/foobie_bletch.sql] FAIL
L:   6 | P: 101 | L016 | Line is too long.
L:   8 | P: 112 | L016 | Line is too long.
L:  10 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  11 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  12 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  13 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  14 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  15 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  16 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  17 | P:  47 | L011 | Implicit aliasing of table not allowed. Use explicit
                       | `AS` clause.
L:  18 | P:  63 | L011 | Implicit aliasing of table not allowed. Use explicit
                       | `AS` clause.
L:  19 | P:   3 | L003 | Indentation not hanging or a multiple of 4 spaces
L:  19 | P:  45 |  PRS | Found unparsable section: '\n-- need this because there
                       | can be multi...'

[sethwoodworth]

I took a pass at parsing the YAML output format and am having similar issues:
let &efm='-\ filepath:\ %f,%E\ \ - code: %t%n,%C\ %#line_no:\ %l,%C\ %#%l,%C\ %#line_pos:\ %c,%C\ %#description:\ %m,%C\ \ \ \ \ \ %m'

On input of:

- filepath: models/foobie/applications_a.sql
  violations:
  - code: PRS
    line_no: 19
    line_pos: 45
    description: 'Found unparsable section: ''\n-- need this because there can be
      multi...'''
  - code: L003
    line_no: 10
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L003
    line_no: 11
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L003
    line_no: 12
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L003
    line_no: 13
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L003
    line_no: 14
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L003
    line_no: 15
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L003
    line_no: 16
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L003
    line_no: 19
    line_pos: 3
    description: Indentation not hanging or a multiple of 4 spaces
  - code: L011
    line_no: 17
    line_pos: 47
    description: Implicit aliasing of table not allowed. Use explicit `AS` clause.
  - code: L011
    line_no: 18
    line_pos: 63
    description: Implicit aliasing of table not allowed. Use explicit `AS` clause.
  - code: L016
    line_no: 6
    line_pos: 101
    description: Line is too long.
  - code: L016
    line_no: 8
    line_pos: 112
    description: Line is too long.

[FelipeLema]

for anyone interested: I made a wrapper on sqlfluff that can be used with neomake. it prints one error per line

https://codeberg.org/FelipeLema/sqlfluff-oneliner