Add s:mru_list variable.  Add files to it on BufEnter
Add PruneMruList() function and g:niffler_mru_max_history
Add WriteMruCacheFile() function
Modify PruneMruList() to remove duplicate MRU entries
Add UpdateMruList(). Change au from BufEnter to BufLeave.
FilterCandidatesList - use both external and internal filtering

- Use `find` command for external filtering of the filesystem
- Use `vglobal` for internal filtering of the displayed candidate list
- Necessary for MRU search which cannot use the `find` command
Add RefreshCandidateList, used for internal filtering
Break OpenNifflerBuffer() into ONB() and SetNifflerText()
Add NifflerMRU() function and command
Modify OpenSelection() to work with a NifflerMRU search
Reverse mru_list before pasting in the Niffler buffer, preserve mru order