Allow matching on the base branch

[gdostie]

Adding the necessary logic to match on the base branch (issue https://github.com/TimonVS/pr-labeler-action/issues/27).

Side note

I had to update the matcher package to version 4, otherwise an empty array of patterns for the base branch would match any label.

With version 2: matcher(['master'], []) returns ['master']

With version 4: matcher(['master'], []) returns []

Also, I couldn't switch the matcher function for isMatch from the same package because of this bug.