ENH: Allow to influence the file list format via g:MRU_Format configuration.

I prefer the old full absolute filename listing. By exposing the formatter and parser of the file list as a configuration variable, the old behavior (as well as any other personal preference) can be restored:

let g:MRU_Format = {
\   'formatter': 'v:val',
\   'parser': '.*'
\}
Add additional mappings and aliases.

Alias <S-CR> for o and <Esc> for q.
Add <C-CR> to open a file as the :only on.e
Issue error when attempting <C-CR> (open :only) on multi-line selection.
ENH: Add p mapping to open the file in the preview window.
Issue error when attempting preview on multi-line selection.
Merge branches 'enh-open-in-preview', 'enh-mru-format-configuration' and 'additional-keys'