'Fuzzy' file finding -- search on subsequence

(Note: leave for September branch)

This implements a 'fuzzy' finder for files through the `CTRL + .` file finder -- we now match on subsequences, rather than just the start of strings.

This allows, for example:

`rcle`

to match

`roclet-collate.R`

A scoring system is used to weigh matches as well, so the most likely intended results go on top.