chore(.gitignore): ignore release .zip files

For some reason, they weren't ignored. This caused the latest
release build in Travis to wipe those files, so we couldn't upload
them to the GitHub Release.
See https://travis-ci.org/injoin/frontkit/builds/34651483#L763
feat(navbar): define bg for nav link on hover

Added @navbar-link-hover-bg variable that allow us to define a
background color for a nav link on hover.