Make it possible to configure multiple flags by defining an array of flags

Defining multiple flags like this didn't work:
flags: '--format=pretty,junit --out=build/behat'
it gave the following error 'Too many arguments'

the args object you give to the spawn function every argument should be an item in the array.

So with this PR you can define multiple flags like this:
flags: ['--format=pretty,junit', '--out=build/behat']
The default way of defining a single argument will also still work.