Make all mappings silent

Closes #123.
Expose "open buffers" functionality for mapping

References #123.
Tests: add support for NeoBundle numbness

Closes #122.
Doc: use autocmd for remapping '<cr>'

Closes #126.
Refactor show_dir and show_files

This de-duplicates the shared code, and adds some (first) improvements
to not stat all of v:oldfiles (twice).

If accepted, it allows for easily adding a cache for abs_path handling
etc.

 - Add _show_filtered_oldfiles and _get_filtered_oldfiles
 - Fixes g:startify_files_number<1