Allow single quotes for Strings to allow parsing JSONPath

[JanHoefelmeyer]

As indicated in PaesslerAG/jsonpath#38, Gval has trouble evaluating some JSONPath expressions since its syntax in that point conflicts with Go syntax:

Strings in Go are portrayed by double-quotes "..." and as
raw-strings ... . Single-quoted-strings '...' are not known in Go. Instead, they are used to indicate single characters and runes. This means, when Gval encounters single-quotes strings, it will try to parse it as a single character and fail, throwing an error.

This is because Gval uses the text/scanner as is. To solve this issue, there could be an option to allow for alternate parsing, which would take single quotes or similar into account. Instead of the text/scanner package, a custom, similar lexer could be used.

Addtionally, the strconv.Unquote functionality is also based on Go, and as such will not accept single-quoted strings. As such, the parseString-method could be adjusted as well, so that it transforms single-quoted strings into double-quoted strings, allowing them to be parsed.

These changes could be made optional as to not disrupt potential workflows that already depend on jsonpath not evaluating single quoted strings.

[bernhardreiter]

This issue with the corresponding PR#98 is a precondition for fixing PaesslerAG/jsonpath#38 .

@mhengl if you have questions, please let us know! :)