Add Idris.

This adds Idris into the mix and uses the text-only parser for now, pending
upstream merging this patch in:
https://bitbucket.org/birkenfeld/pygments-main/pull-request/210/idris-lexer-added-lexer-for-idris/diff

Once that gets merged in, the lexer should change to idris.
Merge pull request #751 from CodeBlock/master

Add Idris.