Custom Reader View shortcut wót plucafs

This extension unlocks the ability to edit the shortcut that triggers the Reader View.
The default shortcuts (CTRL+ALT+R/F9) will remain available.
When the addon is installed, a new shortcut is added: ALT+A.

How to change the shortcut:
If you want to change it, these are the steps:
1. Open `about:addons` (paste it in the address bar). It'll open the Firefox's settings page, specifically in the Extensions page
2. Click on the little gear on the top-right and select `Manage Extension Shortcuts`. It'll open a page listing all the extensions that have defined at least one custom shortcut
3. Look for the `Custom shortcut for Reader View` and edit the default shortcut ALT+A with your preferred one

How it works:
An extension, in the file that describes the overall extension (manifest.json), can declare multiple custom shortcuts. Then, they'll be read by Firefox and freely edited in the page opened previously. In the extension I added a simple function (triggerReaderView) that triggers the reader view when the custom shortcut is pressed.

How to get the source code:
You can right-click on the button used to install the extension, and click on `Save link as...`. It'll save a .xpi file that, after extracting it, will give you access to the source code.