Patch release: Check existing Trello links before adding one

Fixes

• #3 Check Trello card link existence before adding new one

Adds links to PR and issue (a comment) to Trello card

v1.1.0

Release v1.1.0

v1.0.0

Release 1.0.0

You can

• create a Trello card when new Github issue is created (syncs matching labels, creates backlink to the issue)
• move card when you create/update/merge/... a pull request which refers (#1234) to an issue in the issue description

Cards to be moved must have backlink to the Github repository PR belongs to.

While Github Power Up is not required in Trello it makes Trello UI to pull more data from Github issues and pull requests.