list bundled plugins to use with -h

[arlolra]

as discussed

[baudehlo]

Merged, but also if supplied with -c should it list the local plugins?

Further to this line of thinking, we could also do "haraka -c

--create-plugin <plugin_name>" (or some better parameter name since --create is taken), and it could create a template plugin file, and a local docs/plugins/plugin_name.md file...

[arlolra]

see #13

i like the idea of a convenience method to start plugins
but i'll leave that to you

what do you think about generating a package.json similarly to your README?

[baudehlo]

Yeah a package.json would probably make sense.