action.yml missing `theme-name` config var

[Mark Woosey]

theme-name does not appear in action.yml and so causes a warning when run as follows:

Unexpected input 'theme-name', valid inputs are ['api-url', 'api-key', 'exclude', 'file']

[Mark Woosey]

As per #23, the key is used by the action, but missing from actions.yml

[Daniel Lockyer]

Fixed by #24