Adding support for roundcube

[liliakai]

Roundcube is the hosted webmail client for riseup users. In this client, raw .eml files can be downloaded from a particular url constructed using a special token associated with the message, and this token can be found in the DOM, which means support for this client can be added in the same way that gmail and yahoo are implemented:

1. Use regex or DOM element selection to search for the relevant token
2. Construct and fetch the raw email download url, ex:

 https://mail.riseup.net/rc/?_task=mail&_save=1&_uid=1&_mbox=INBOX&_action=viewsource&_token=MESSAGE_ID

See also: relevant notes in CONTRIBUTING.md and examples in contentScript.js.

[mfc]

👍 you're a boss! thanks for investigating it and mapping it out for future dev work.