Add evil keybindings through a idris-mode hook

This fixes #272.

I think calling evil-leader/set-key-for-mode in a hook is not idiomatic, but then doing otherwise would mean either

1. changing the way other keybindings are defined to add to idris-mode-map rather than using hooks (that might be more idiomatic too, I'm not sure) so that all keybindings are defined in a more "declarative" manner and customized differently than now; or

2. treat evil keybindings in a special way (with they own custom variable).

The former would mean users would have to redo customizations they might have done. Let me know if you'd prefer something else than what I did in the PR.