Feature: Allow passing in custom patterns by passing in github repo

Would be great to use this for patterns other than those specifically in cdk-patterns (btw thanks for all the awesome cdk patterns stuff!) Any interest in this feature? I'd be happy to implement. 

potentially something like:
```
npx cdkp init corrjo/devpattern
or
npx cdkp init --external corrjo/devpattern
or
npx cdkp init-external corrjo/devpattern
```