Textern per Jonathan Lebon

**IMPORTANT**: this add-on currently only works on Linux and requires
Python 3.5 or higher.

Textern is an add-on that allows you to edit any textbox using your
favourite editor, such as vim, emacs, and sublime!

You simply have to select the textbox you want to edit, press
Ctrl+Shift+D, and let Textern open your editor.

It is similar in functionality to the excellent "It's All Text!" add-on:
https://addons.mozilla.org/en-US/firefox/addon/its-all-text/. However,
it uses the WebExtension API and is thus fully compatible with
multiprocessing and expected to work beyond Firefox 57. The downside is
that it requires the native part of the add-on to be installed
separately. This is because the WebExtension API has no support for
directly starting external applications.

To install the native app, you can follow the instructions at
https://github.com/jlebon/textern.

How to use:

1. Install this add-on.
2. Install the native app.
3. The default editor is set to gedit; you can change it to your
favourite editor in the add-on preferences.
4. Select the text area you'd like to edit and press Ctrl+Shift+D and
your external editor will open.
5. As you type and save the document, the text area gets updated with
your text.