Follow XDG base directory specification

**youtube-dl** already looks for `XDG_CONFIG_HOME`, so it should also look for `XDG_CACHE_HOME` and use it if set, according to the [XDG base directory specification](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html).

I changed **youtube-dl**'s behavior to use
```
$XDG_CONFIG_HOME/youtube-dl/config
$XDG_CACHE_HOME/youtube-dl
```
if the environment variables are set, otherwise use the corresponding one of
```
~/.config/youtube-dl/config
~/.cache/youtube-dl
```