Specify branches to exclude from trigger map

Description of the feature request

Context: https://bitrise.slack.com/archives/C03E2R9A4/p1553970021009000

The trigger map should allow branch exclusion, e.g. !branch_name to trigger a certain workflow for all branches except branch_name.

Use case / for what or how I would use it

We have a branch called l10n_develop where translations are synced back from the localization service once translators update a string. The service creates a new commit for each language, creates a pull request, and then pushes a new commit for subsequent changes (see https://github.com/iotaledger/trinity-wallet/pull/1222 for an example). This can result in a lot of builds being created even though we don’t need our tests to run on these changes.

Thanks for the #feature-request @rajivshah3!

Makes perfect sense, don’t forget to vote for it!