Add keyword trigger condition

[kdumontnu]

This enables a new trigger for keywords in the commit message or title. The usage is as follows:

trigger:
  keyword:
  - "*[RUN PIPELINE]*"

Should only run if the keyword is matched.

or

trigger:
  keyword:
    exclude:
    - "*[SKIP DEPLOY]*"

Should not run if the keyword is matched.

I considered simplifying the notation by appending * to each end of the keyword strings, but it would give the user less control on matching specific strings.

Note: The way this is currently implemented follows the same syntax as other triggers, but requires adding the Keyword condition to drone-yaml. However, I see that the GitHub repository is currently archived, so I'm unable to open a PR.

On further inspection it looks like drone-yaml Conditions essentially expects each argument to be a "path" in the matching operation. Passing an arbitrary string (message or title) will probably result in odd behavior. The solution here appears to be to simply create a separate Match function for Condition that uses go strings.Contains and remove the wildcard operators above. Again, this is in drone-yaml, which appears to be private.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[kdumontnu]

Just saw this issue, which appears closely related to this. Seems like there was no consensus on the proper syntax, so I propose to leave it up to the user which strings to match.

[tphoney]

Hey @kdumontnu this seems to have went a little stale, there are a few things i would like to raise. It is unclear how this would work and it needs a little bit more explanation. I would suggest creating a proposal for this feature here https://github.com/drone/proposal first . So we can gather feedback on this would work. I am closing the issue for now, due to staleness not signing the CLA and failing tests.