allow command line arguments, limit help regex matching

* change the "^help" regex to "^help$"  so other plugins may also start with the word help, i.e. "help me do XYZ" without also matching the general "help" command.
* help response is now organized by plugin class

NO LONGER IN THIS PR
--- start_will.py and start_dev_will.py accept optional arguments to override plugins and templates directories. ---
```
./start_dev_will.py --help
usage: start_dev_will.py [-h] [-p [PLUGINS]] [-t [TEMPLATES]]

Start Will the Chatbot

optional arguments:
  -h, --help            show this help message and exit
  -p [PLUGINS], --plugins [PLUGINS]
                        comma seperated plugin directory paths (default:
                        ['plugins'])
  -t [TEMPLATES], --templates [TEMPLATES]
                        comma seperated template directory paths (default:
                        ['templates'])

```