Autodetect mode

[hinell]

I request to make a general API that detects current user mode (normal, visual or insert) automatically by using built-in [nvim_get_mode()](http://localhost:5001/doc/user/api.html#nvim_get_mode()) API so we don't have to bind separate keys for moving a line or chunk of text (in visual/block mode).

Thanks!

[fedepujol]

This is has low priority for now. I'll implement a mapping config on the setup function.