[wip] feat: Add the ability to auto merge PRs

[sambhav]

This PR adds a couple of things -

1. A new event type listener for status.* this allows us to listen to status change events so that we can fire the merge action on status change events.
2. A validator for status.*. This validator resolves the commit related to the status to relevant PRs on the repository. There is a checks setting option it takes to filter our PRs with failing checks. (This is optional as some checks may not be required to do merge)
3. Adds support for status* events to the merge actions.
4. Checks that the pull request is not in a blocked mergeable_state before trying to merge. This ensures that the PR is ready to merge (passes required checks including status, review and other branch protection checks)
5. Fixes broken merge action. merge action was missing the name parameter.

Fixes #395

Fixes #386

TODO: Update docs and add tests.

[sambhav]

@shine2lay @jusx for some reason the tests are failing with node 8.8 but work with node 13.10 - thoughts? (Which is why I updated the node version)

[jusx]

@samj1912 Could it be that the code base uses JS features that is not supported in node 8.

[jusx]

@samj1912 I like the approach and it looks good to me. I noticed a TODO note in the description. Will you be adding the docs and test as part of this PR? Thanks in advance.

@shine2lay Please take a look at this. It looks like it will work well and will resolve #395 and probably #386

[sambhav]

Note - I have a much simpler WIP in mind. But it relies on #402 being closed.

[sambhav]

Closing this for now - will add a better version of this soon.

[mergeable]

❌ Error Occurred while executing an Action

If you believe this is an unexpected error, please report it on our issue tracker: https://github.com/mergeability/mergeable/issues/new

Error Details

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Error: Merge failed! error : HttpError: 2 of 2 required status checks are expected. At least 1 approving review is required by reviewers with write access.