Feature/installer

Allows a user to install plugin requirements without installing from source.
Ah, I see. I would keep all CLI argument validation and program exit code outside of the helper functions. You could put this validation in `main()`:

```
def main():
    # ...
    args = parser.parse_args()
    if args.command == 'install':
        if not args.plugin:
            print('Must provide at least one plugin name to install.')
            sys.exit(1)
        plugin_requirements(args.render, args.export, args.plugin)
```

Then remove the check in `plugin_requirements`