Support XDG basedir spec

[qfox]

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

Why .bemconf.js?

[tadatuta]

code highlight, linters, all these things

[qfox]

When we decide to cleanup all this trash?

Why we walking on the streets with known rakes?

[tadatuta]

What's your proposal? :)

[qfox]

For resolving algorithm, look for each directory first match by name:

• .config/bem - local in the root of current directory as file bem (if bem is a file)
• .config/bem/common.json - or local ... as the file common.json inside bem directory (if bem is a directory)
• .bemconf.json - also, we can make a simple .bemconf.json file for nested directories only.

And the same for per-user configuration, but with ~/ at start (looks like we don't need .bemconf.json here):

• ~/.config/bem
• ~/.config/bem/common.json

And for per-server conf (we don't have this atm):

• /etc/bem
• /etc/bem/common.json

Besides, there are XGD_CONFIG_HOME and other env variables, that should be used for basedir convention (xdg spec).