Add XDG Base Directory support.

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

(that’s .config, and friends)

Fixes #84
Support PSYSH_CONFIG to specify config file.

Calling psysh with `--config` wins, of course.

See #84
Usage should contain only the basename.
Update PsySH Compiler for Xdg, new PhpParser.
Add missing $readline Configuration private member.