Plugins rewamped

This fixes the issue presented in #16 to some extend.

This means that plugins have to register with a simpler function. Here is an example for a new command:

```python
from beetle import Commander

def some_function():
    print('display! display!')

# This function is called by beetle, after importing the module.
def register(plugin_config, beetle_config):
    Commander.add('command', some_function, 'help! help!')
```

It appears to be lot cleaner, than having a long, loooong list of parameters you have to accept, but not deal with.