Add github token support

[mattstratton]

Token should be optional but suggested (need to add this to documentation)

Environment variable should be called BOWIE_GITHUB_TOKEN

Set it by

export BOWIE_GITHUB_TOKEN="«your-40-digit-github-token»"

Can also take token flag (shortcut t):

--token <your-40-digit-token>

[mattstratton]

Actually, let's not make it optional. It should be required because that will make the code cleaner.