Support Dotenv with quotes and nested quotes

[carlthuringer]

Addressing #97

When parsing dotenv files, if there's any guide to follow, it's the defacto npm dotenv project

The parsing engine currently supports the following rules:

• BASIC=basic becomes {BASIC: 'basic'}
• empty lines are skipped
• lines beginning with # are treated as comments
• empty values become empty strings (EMPTY= becomes {EMPTY: ''})
• single and double quoted values are escaped (SINGLE_QUOTE='quoted' becomes {SINGLE_QUOTE: "quoted"})
• new lines are expanded if in double quotes (MULTILINE="new\nline" becomes
  {MULTILINE: 'new
  line'}
• inner quotes are maintained (think JSON) (JSON={"foo": "bar"} becomes {JSON:"{"foo": "bar"}")

This PR implements a more sophisticated matching using a regular expression. This allows the parser to handle all the cases presented in the dotenv node package.

[carlthuringer]

I'm no groovy/java developer, so I can't actually figure out the android code. I tried several ways to test JAVA regexes online but no real results. I think someone who knows how those work can easily take my pattern and apply it to the gradle script.

[janoist1]

You can escape quotes, that helped me passing JSON in.

[pedro]

You rock, thank you!

[pedro]

Ah hmm testing some stuff before releasing I noticed this dropped support for unquoted values (the 1st example in https://regex101.com/r/SLdbes/1). I made the quotes optional for now to not break existing apps. Do you think we should explicitly ignore these?