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': '.*'
\}
```