always enclose prospective submissions in new <a>

When using anchor tags to enclose a prospective submission, we should
never attempt to apply our class on existing elements. Instead, we
should always create a new wrapper anchor element.

The reason is so that we can treat these anchors like <span>s. We can
safely use 'inherit' for the properties we care about because we're
guaranteed for our new anchors to be children of the original source
elements.
adjust anchor styling to mimic <span>s

Fixing more styling issues. In particular:
- enforcing that padding and margin should be 0
- font-size should always match the surrounding text
- we do not want to change the initial vertical-align value
simplify anchor v. span branch
supply local chrome extension to run interface

If a webserver is running to host the local development files, then the
provided Chrome Extension will inject all scripts and the source
betterlink-no-interface.js file into visited webpages.

A Rack configuration file is provided to run a sample webserver via
`rackup -p 1000`.
add Anchor Highlighter dependency
fix assignment of cleanup() function

It's not as explicit that a cleanup function is exposed, but it's more
reliable to directly apply the function assignment during
initialization, instead of through a proxy variable.