Allow to specify custom module name passed to babel-plugin-react-intl

[Filson14]

New option enabled to define module name from where elements defining messages are being imported (instead of default reacti-intl). It is passed to babel-plugin-react-intl

Why:
In case of using custom module based on react-intl, importing defineMessages from it successfully defines messages for an app, but later extraction is not possible without feature mentioned above.

Checklist:

• Documentation
• Tests
• Ready to be merged
• Added myself to contributors table

I was wondering, if adding only moduleSourceName option from all babel-plugin options is OK or not. I think other options are not important (or even do not make sense) in case of extract-react-intl context.

What I mean is:

• messagesDir - the tool returns object on runtime, do not generates files
• enforceDescription - extractor omits descriptions, so I guess it would not be helpful
• extractSourceLocation - returned object is flat JSON structure with 'key - message' properties, so additional information would be omitted as well

I wait for other opinions, so if someone consider those needed, I can update the code.

[akameco]

@Filson14 I think that moduleSourceName alone is enough.

Thank you for your PR. Merge.