Source files json in pretty format with line breaks

[donquixote]

See also #176. I cannot reopen that, so I am creating a new one with a (hopefully) more clear description.

Source maps are generated in an "ugly" json format, without line breaks or indentation. Very much like a minified CSS file.

This may be good enough for most cases, but there are some situations where a pretty format would be more useful:

• If the source map is checked into a git repository. git diff, merge etc works much better if the code is broken into multiple lines.
• If a developer is actively looking at the source map code.

Usually the main benefit of a non-formatted file is performance, by slightly reducing the file size.
This is not really relevant here, because the file will only be downloaded if someone opens the browser developer tools.

Current format:

{"version":3,"sources":["overrides.css","overrides/_forms.scss","overrides.scss","variables/_typography.scss", [..]

Pretty:

{
  "version": 3,
  "sources": [
    "overrides.css",
    "overrides/_forms.scss",
    "overrides.scss",
    "variables/_typography.scss",
[..]

This should be optional, so that existing code does not change unexpectedly from downloading a new version of gulp-sourcemaps.