ag-typed function. Version, where one can choose file type to search.

Thank you for response. I tried to satisfy your review, and also changed ag-typed
behaviour to search according to current buffer type, old function was renamed to ag-with-type
I belive s and dash are modern libraries, that show green light to emacs newbies as me,
so it could be a nice step to use them, and maybe to include in standard emacs later.