Add capability to filter tests by string

[Jean85]

As per #81, this PR adds an optional argument to the commands to filter the tests.

This works as a superset of PHPUnit's functionality, since:

• is always case insensitive
• it works in combination with --testsuite (PHPUnit ignores that)
• it searches a match everywhere in the filename, so it doesn't have to be a full or relative path

I will add this fully to the docs later.