Append to Clipboard per Brett Zamir

Simple add-on to append to, rather than replace, current contents of the clipboard.

Choices include no separator between appends or single or double line breaks.

To ensure plain text, you might consider using https://addons.mozilla.org/en-US/firefox/addon/copy-plain-text-we/ or https://addons.mozilla.org/en-US/firefox/addon/copy-plaintext/ .

Also optionally adds a "clear clipboard" menu item.