refactorings after of review comments

So, since you nicely asked for review, and I had some time to spare (well, actually not this much, but I got hooked), please find here my ideas.

The findings
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So what did I find:

 * no namespaces
 * no const correctness
 * no member initialization
 * C style interface with pointers everywhere
 * no exceptions
 * no exception safety
 * redundant counts for vector (and racy at that)
 * broken relational operator on tinydir_file
 * errno being assigned to
 * gratuitously reading directory **twice** in tinydir_open_sorted
 * assigning false to size_t
 * function definition in header file, no header separation for tinydircpp.h

 * bad interface design (unclear naming, conflicting goals/usage patterns)
 * inefficient design (no iterator/callback interface, forcing allocation of all dir entries up front)
 * ibid.: path duplicated across all tinydir_file objects

 * lack of guards (assuming filenames without path separators, not checking for embedded NUL chars)
 * buggy string handling: setting `path[path.size() - 2] = '\0';` instead of resizing
 * bad logic (checking for file extensions matches in the middle of names as well)

 * bad style: having local variables with the same name as member variables and with slightly  different meaning
 * bad style: many redundant pieces of code (clearing files, closing dirs, setting paths)
 * bad style: implicit indirect includes (algorithm for stable_partition)

This is outside the API reservations I had noted with this commit: https://github.com/iamOgunyinka/tinydircpp/commit/a484a041f1f918d15fad42e22a27e878fb652e42#commitcomment-5248483

So, without further ado find my pull request that I tested on linux... It **should** still work on Windows.

Now, there is 

 0. no more C artefacts (bail, close, tinydir_file* etc.)
 0. no more `vector`; no longer a requirement to store all files in a big container first
 0. much more flexible interface (filter for heterogenous file types at once, filter for other characteristics than the extension).
 0. vastly reduced public interface
 0. 28% code reduction (145 LoC reduced), much cleaner headers

Remaining issues I can think of now would be

 * FileHandler is now a class with too few responsibilities. Each member function could be a static function that just takes the `source` directory as well
 * Error handling should be improved (right now, encountering (broken) symlinks will throw e_noent)