Separate plugin config file

Hi Evan

Here's my PR WRT https://github.com/evantahler/actionhero/issues/512. It's a branch obviously, I thought that'd likely be best but I can merge into `master` if you prefer.

I hope i've done the right thing but please let me know if you don't think I have. It's tricky to test without being able to install via NPM but from what I can see, the plugins are loading correctly.

I wasn't sure whether to add to `package.json` `install` string or not but this PR includes the option for an argument on the `generate` method to add plugins to the new `config/plugins.js` file. The arg is `--plugins` and just uses the existing `binary.argv` object. My thinking is that this doesn't break any existing script people may be using and does not add plugins by default but does allow programmatic addition of plugins via a little shell scripting or similar.

Hope that's all OK anyway. If so, I can perhaps update the `gh-pages` branch to include the info on the flag and the config file.

Cheers
Neil