nitrogen_dev (bin/dev command): add 'plugin' command to create a plugin within the current deployment.
nitrogen_dev (bin/dev command): add 'plugin' command to create a plugin within the current deployment.

This commit expects plugin prototypes added to the nitrogen/ repo.

$ bin/dev plugin xxx
Plugin created in ./lib/xxx (demo page in ./site/src/xxx.erl)

Don't forget to:

1) initialize your newly created plugin with version control (hg or git)
2) clone your plugin outside this nitrogen release and update the 'deps' section in your rebar.config. For example:

   {my_plugin, ".*", {hg, "https://bitbucket.org/user/my_plugin", default}}

3) recompile and check the demo page /xxx

nitrogen/nitrogen_core,choptastic/nitrogen_core,nitrogen/nitrogen_core,nitrogen/nitrogen_core,choptastic/nitrogen_core,nitrogen/nitrogen_core,choptastic/nitrogen_core