Add Flow language

[jayvdb]

Flow language is one we need for coala-bears.

c.f. coala/coala#5343

[utkarsh2102]

Hey,
Can I take this up?

[jayvdb]

Do it in coAST first, where we have better ability to add complex relationships between languages.

[utkarsh2102]

Do it in coAST first, where we have better ability to add complex relationships between languages.

Sure, on it.