feat(#27): change status based on existing label

[hemanik]

The current WIP plugin looks for the PR title prefix to set the status. The scope of this enhanced to change PR status based on the existing label.

The new behavior handles the following cases as described below:

• set the status to failure if PR either contains a wip label (irrespective to the title prefix) or it has a wip prefix in the title (in which case the label is automatically added).
• set the status to success if the PR is updated by either removing the wip prefix from the title (in which case the label is also removed automatically) or by removing the wip label (in which case, if present, the wip title prefix is also remove automatically).

Fixes: #27

[centos-ci]

Can one of the admins verify this patch?

[MatousJobanek]

[test]