Add a friendlier “missing pdo sqlite” message.

This could probably still be improved, but it’s at least one step more understandable, so let’s ship it for now :)
Minor CS fixes.
Allow overriding PsySH configuration file.

e.g. 

$ psysh -c /path/to/some/psyshrc.php

Also, display prettier usage info when command line input validation fails. Because backtraces are ugly and not all that useful.

Fixes #64