Implements multiple file license header insertion

[mitchwadair]

Intended to close #32

• Adds new command, only visible in context menu of folders in explorer.
• Recursively adds license headers to all files within a directory/subdirectories
  • Only inserts license header into files which do not contain one already

I tried my best to do this in a similar way to the original _insert function, but was unable to find a good way to go about it. This currently uses the fs module to insert licenses. This may be an inefficient way of accomplishing this task, so I can attempt to do it in a way that has similar behavior to _insert if desired, although it may be a bit uglier.

[ymotongpoo]

Thank you for the request, @mitchwadair ! This is awesome feature to add.