Fix typo in program config setting.
Add helper function to load and parse in one shot.

The lack of monads in C++ unfortunately inflates the amount of helper functions
to chain together monadic calls, *sigh*. Here's a new helper for the sequence
of loading and then parsing file contents: load_and_parse<T>(path p). This
function (i) first loads the file contents, and (ii) then attempt to parse it
as usual with to<T>(string const& str, ...).
Fix potential segfault.
Add directory abstraction to facilitaty traversal.

The new filesystem class vast::directory serves as an interface to directory
enumeration and traversal. It renders vast::traverse obsolete, as it provides a
much nicer range-like interface.