Option to check whether the issue already exists on the project

[c00]

Create an option called ignore-if-already-exists or something more aptly named.

• When set to true, it would check the board to see if the issue has already been added to it, and if so, it would complete successfully without any further actions.
• When set to false, it would check the board to see if the issue has already been added to it, and if so, the workflow fails with a clear message stating the issue already exists.

Currently what happens is that the workflow fails with an uninformative "Validation Failed" error in a situation where the issue already exists on the board.

Use case:
Users use the project board to create a new issue, so on creation, the issue can already be on the board.

[ritchxu]

@c00 check out #14