redirect-to-abstract by stites

* redirect-to-abstract - Firefox extension

Redirect all pdf links to their abstracts, and overwrite abstract pages to
bypass the regex (by either appending an ~&~ or ~?~ at the end of the url).

Trying to keep this codebase dead simple (turns out my last attempt was too
ambitious). Should be chrome-compatible, but someone else will have to test.

Works for the following archives:
- arxiv.org
- openreview.net
- proceedings.mlr.press
- proceedings.neurips.cc

* Building
Requires browserify. Just run ~make~.

* Prior Art
- https://github.com/stites/MyArx -- a conglomerate of a bunch of firefox+arxiv
extensions, forked from https://github.com/j3soon/arxiv-utils . This is where
the icon comes from but I should probably replace it.
- https://github.com/weakish/arxiv-url -- original inspiration but is too
ambitious in that it tries to rewrite all urls on every page. this extension just intercepts
requests to the pdf links and sends to you the abstract.