Introduce new `mergeable` requirement, in similar vein to the `approved`
requirement. Ran `make go-generate` to update mocks accordingly.
This addresses issue #43.
If projects are configured using an atlantis.yaml with a specific
project name, include that in the output for comments. This is useful
because often the project names are better identifiers than the
directory and workspace names and also because some projects use the
same dir and workspace so the only way to differentiate them is via
their names.
Fixes#353