solr: add startup plist

Added a plist startup like was previously done in [solr36](https://github.com/Homebrew/homebrew-versions/commit/4fa8b9b3c8f37667f1d993a53a6b9da383d87179) at Homebrew/homebrew-versions#366.

Defaults to running Solr on port 8984 so it can run in parallel with solr36 which runs on port 8983.
Without the `unless` check I get a warning if the folder already exists:
```
Warning: The post-install step did not complete successfully
You can try again using `brew postinstall solr`
```
And although that will still work I think it is a bit confusing.

I need the `unless ... exist?` because the example files are installed to /usr/local/solr/multicore and are supposed to be changed by the user when adding new/configuring new cores. Configuration that should persist on upgrades.