Add support for XML configuration file

[fbastien]

Feature request:
Could you add an option in the preferences UI so that it's possible to specify the location of the XML configuration file ? I mean I need to do the same as the -c/--configuration argument of the CLI (see https://phpunit.de/manual/current/en/textui.html).

That would be very useful. Thanks.

For instance, at the moment I can't run any of my tests with your plugin because my test classes reference environment-specific constants which are defined in a phpunit.xml file (as in https://phpunit.de/manual/current/en/appendixes.configuration.html#appendixes.configuration.php-ini-constants-variables). When I use the regular PHPUnit CLI it works fine, but every attempt to run my tests with this plugin miserably fails because of undefined constants.
Here is just my particular problem, but supporting such an option would benefit much more than just resolving my issue, as such configuration file provides much more options than those available in the plugin UI.