'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.
Could we use boost string algorithms contains and icontains to implement these? (http://www.boost.org/doc/libs/1_50_0/doc/html/boost/algorithm/icontains.html). Re-using existing code would eliminate the chance of subtle defects in our implementation.