properly open urls from firefox reader mode page

[wesinator]

Thanks for the work on this extension.

This PR adds a check to properly handle the case where the current page view is a reader mode (in Firefox), sending the actual raw URL to resurrect pages.

Enter about:reader?url=https%3A%2F%2Fwww.howtogeek.com%2F352267%2Fhow-to-use-the-reader-view-in-firefox%2F in browser and the resurrect pages from the toolbar links.

[arantius]

Thanks for the PR. But:

1. It's got syntax errors (missing closing paren, decodeURIcomponent is not defined).
2. Coding style doesn't match (indent by two, braces around blocks aren't optional, single quotes around strings)
3. Even if that's fixed, it functions only from the context menu -- the toolbar button behavior is now inconsistent.

(edit: markdown syntax)

[wesinator]

@arantius happy new year,

This change is now fixed, tested and works correctly