danger: Allow scoped merge request titles

In the Git ecosystem, commit titles are commonly scoped to a given subsystem to make immediately clear which parts of the system are being changed (see e.g. this very commit's title). Using the same pattern for merge requests is thus a natural urge for many used to this, but our Danger rules will complain about such commit titles as their first character is not an uppercase on.e

Let's improve the rule to allow optional (lower-cased) scopes.