XDG base directory specification defines where various files
should be looked by an application. Hopefully, this makes `frostfs-cli`
more predictable and pleasant to work with. Luckily for us, golang already
has everything we need in the stdlib. This commit also gets rid of
`github.com/mitchellh/go-homedir` dependency.
Close#532
Refs #1455
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
