[FR] Expose `ParserState` of currently invoked function

[am11]

As per the discussion that transpired at: sass/node-sass#646 (comment)

As an implementer I want to propagate the contextual information, in particular, the file path, line and column numbers to the end-user so that they know from where the function was called.

[mgreter]

Technically this translates to beeing able to access the ParserState of the currently invoked function.
This is pretty much the same as the API for accessing the import stack (just return a new sruct type).

[chriseppstein]

Related: #1227

[mgreter]

This is pretty much possible now, beside fetching line and column numbers!