Adding new components after creation

[stevencolinmartin]

Potentially out of scope but it would be good to add in other bits after creating the initial package.

Things that would be nice to add

• a new node cpp or python
• new msg/service or action

[btalb]

It would be a nice addition, but leaving it in the too hard basket for now.

I do some pretty blunt things to the directory where a template is initialised, and would have to proceed extremely cautiously to ensure I don't clobber any existing files.