Automerging merges pull requests automatically if all plans have been
successfully applied.
* Save status of PR's to BoltDB so after each apply, we can check if
there are pending plans.
* Add new feature where we delete successful plans *unless* all plans
have succeeded *if* automerge is enabled. This was requested by users
because when automerge is enabled, they want to enforce that a pull
request's changes have been fully applied. They asked that plans not be
allowed to be applied "piecemeal" and instead, all plans must be
generated successfully prior to allowing any plans to be applied.
If users have created an atlantis.yaml config for a specific directory,
only allow commands for workspaces that have been configured in that
directory. This will help prevent errors when users mis-type a workspace
name.
Introduce new `mergeable` requirement, in similar vein to the `approved`
requirement. Ran `make go-generate` to update mocks accordingly.
This addresses issue #43.
