Have MDK specify alternate Behat browsers #163

It would be handy if we could more easily allow for selection of a different browser.

In terms of config, we just need something like:

$CFG->behat_profiles = [
    'chrome' => [
        'browser' => 'chrome',
    ],
    'firefox' => [
        'browser' => 'firefox',
    ],
];

$CFG->behat_profiles['default'] = $CFG->behat_profiles['chrome'];

And then to run, we need to specify an additional argument when running behat:

--profile chrome

The existing behat addConfig commands in MDK only deal with a line at a time.
I'd like to suggest we perhaps add a line to require_once another config file (stored within MDK somewhere) to help with this.