Add ability to discard tabs

[mikeconley]

Via this API: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs/discard - compatible with Firefox 58+.

[tamedfox42]

Hi @mikeconley! I worked on this issue and opened pull request #32 to merge my commits. I cannot assign reviewers in this repository, so would you like to review my changes or assign it to someone else?

Thanks!