support nested directories in template directories

[chinatsu]

i've been wanting something like #66 for a while.

my use case specifically is qbittorrent-themes.
they need a file called config.json (by that specific name) which is pretty generic.
i could post-process filenames before i run it through rcc, but i thought it would be nice for pywal16 to support child directories inside the template folder instead.
this way i can organize my qbittorrent-stuff and have room for more config.json files that might be needed for other programs.

the PR introduces a naive class which contains every reference needed to read templates and output them to the correct spot. this could also just be a tuple, or a dictionary or something if you prefer that.
walk is a super simple function that leverages os.path stuff. i went with a generator-type of function, idk if you like that lol.

i was thinking about defining some tests to verify its functionality, but it seems the tests use my personal config, so it's hard to make a test that will work in every environment.
running the export tests locally, i can at least verify that feature parity is maintained.

[eylles]

okay i like how this looks