Resolves #157 without changing default behavior

Now accepts an optional argument, eg:

```bash
$ make build doc=authors-interfaces.md
PYTHONPATH="/home/charles/workspace/juju/docs/tools/plugins:/home/charles/workspace/juju/docs/tools/plugins/gfm:" tools/build-from-source authors-interfaces.md
Converting authors-interfaces.md
```

I think having a single target is fine for the build action, however I'm +1 for adding a watch target, and already have phaux solution in place for that.

`pip install watchdog`

then follow that with

```bash
watchmedo shell-command   --patterns="*.md;"   --recursive   --command='make doc=$(basename ${watch_src_path})' .
```

Resolves #157