Add bash script to concatenate video files.
concatenator: Substitute bash script by python implemenation.

It may be more robust, since we don't have to care about the number of
prepended zeros in the name of the files, as long as the files can be sorted
lexicographically to get our desired order (using it with `youtube-dl`'s
`-A` option does this).
concatenator: Replace os.system with subprocess.call

This is a more robust solution to avoid having the shell interpret (possibly
incorrectly) what we mean in the script.

As two desirable side-effects, we can invoke `concatenator`:

* Inside a directory whose name has spaces.
* With files to be appended that happen to have, say, parentheses in their
  names.

And probably many other things that I have not yet experienced.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>