Prepros auto-reload extension for Firefox.

Firefox extension for Prepros, fixes #17. Addon is created using [Mozilla Add-on builder](https://builder.addons.mozilla.org). Note that extension is still in Beta and is not tested for every use-case but is good to go in normal usage. While addon is currently in preliminary review and will take 3-4 days to appear in Mozilla Addons Page, it can be installed directly from [here](https://addons.mozilla.org/en-US/firefox/addon/prepros/). Be sure to update Prepros homepage to include the links. :)