Handle backticks in _.escape/unescape

Fixes #1653

Matches [`Handlebars` escape map](https://github.com/wycats/handlebars.js/blob/88c52ded2e81b4b01d12c16e337b0bc4a453a813/lib/handlebars/utils.js#L4-11)
Could I get you to take a peek at the commit I just pushed