Re-implements window.crypto.signText and exposes it to content
Install the Firefox Add-On SDK following the directions
here.
Then, after checking out this repo, use jpm run to run Firefox with the
add-on installed or jpm xpi to package the addon. After the add-on has been
installed, content scripts that call window.crypto.signText should "just
work".
Add certificate filtering by CA name as in original signText implementation.