Feature/installer

Allows a user to install plugin requirements without installing from source.
Minor detail, but you should use the singular `'plugin'` so that the usage spec displays correctly. It currently displays:

```
usage: mfr [-h] [-e] [-r] command [plugins [plugins ...]]
```

What you really want is 

```
usage: mfr [-h] [-e] [-r] command [plugin [plugin ...]]
```