Support RGB, hex values, and add known colours to DC Webmail Notifier

This PR:

* Renames `send` to `send_data`, to avoid overriding the Ruby's `send` method.
* Adds a `colour_rgb!` method (usage: `colour_rgb!(255, 0, 255)`)
* Adds a `colour_hex!` method (usage: `colour_hex!("00ff00")`)
* Adds methods for the following colours: `black`, `white`, `green`, `red`, `blue`, `orange`, `teal`, `yellow`, `pink`
* Does not break the existing WebMail Notifier API (all methods before this change are still supported)