Add config.cachedir to mirror config.configdir and config.datadir

[ethanalker]

Apologies if I'm missing something, but it seems that there is no way to access the qutebrowser cache directory from config.py. The config and data directories were added in #1419, but the cache directory was omitted.

My use case for this is clearing all webengine data (cookies, cache, etc) when reloading the config.

Thanks for all the great work on qutebrowser!

[The-Compiler]

I don't think your usecase will work, for what it's worth (unless you mean on the initial start) - pretty sure Chromium won't like just having its data deleted.

That being said, seems fine by me to expose it for consistency.