Add optional `--name` argument to `drone build` command

We have a home-grown CI system that handles taking commit hooks and kicking off test runs via either local scripts or Test Kitchen, and eventually I would love to be able to replace it entirely with drone.  

In the meantime, though, I was to integrate the drone command as a builder for apps with service dependencies.  With the new caching code to speed up bundles, `drone build` seems like it will work great for us.  

Our system, though, clones the repo into a temporary directly for each checkout. That means our directory name is arbitrary and not consistent for sequential builds.  This pull request adds a `--name` argument to `drone build`, allowing us to specify a `Repo.Name` instead setting it to the directory path. If `--name` isn't provided, we just use the directory path as before.

Does this seem like a sensible approach?