Add Monorepo Support (multiple `start` values)

[daedalus28]

First off, thank you for all your amazing work on this tool - it's a huge help!

It would be super helpful if license-checker could handle multiple start paths for monorepo support. At the moment, I'm iterating through all the directories in a node script and manually combining them, but I imagine this would be helpful to others if support was built in as I imagine monorepos are pretty common.

[stevemao]

Lerna support would be really great!

[matzeeable]

Can you please show an example how you do this?