Extension source viewer per Rob W

Adds a button to right of the address bar when a Firefox add-on or Chrome extension is detected. This also includes extensions for the Opera and Edge browsers and Thunderbird.
This button shows two options:

- Download extension as zip file
- View source

The default action (showing the above options) can be changed via the "Primary action on click"
menu that appears when you right-click on the extension button.

It also adds a context menu item on Firefox and Chrome extension links.

The "View source" option opens a new tab with a simple viewer, with the following features:

- Download-as-zip and download-as-crx at the upper-right corner.
- List of file names, and the option to filter files with a regular expression.
- Find files containing a string, or with content matching a regular expression.
- Quickly jump between search results, or from/to a specific line.
- Automatic beautification (formatting) of code
- Syntax highlighting
- Image preview
- Show hashes (md5, sha1, sha256, sha384, sha512) of the file content.
- View content of embedded zip files.
- Download Chrome Web Store extensions for a different platform (e.g. Chrome OS or NaCl).
- View the contents of any URL or zip file.
- Get permalink to file and search result within a zip or extension file.

Also known as: crxviewer and CRX Viewer.