Given language extensions as parameter or detect them from `.cabal` file

[mgajda]

The current lexer may fail with some non-standard extensions.
One way out would be to be able to pass list of extra extensions,
or detect them from Cabal file.

[mrkkrp]

Which extensions are problematic?

[mgajda]

I ran it on problematic codebase and seen few of them that were added since previous version.
May be good idea run it again and post the list of these extensions here.

[mrkkrp]

May be good idea run it again and post the list of these extensions here.

Please do!