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)
You have to add it to this list: https://github.com/jashkenas/underscore/blob/4f3bd3a666ae42772b43aca0042316c1e7357f7e/test/.eslintrc#L8-L13

I didn't go through and add them all when I created that configuration. Feel free to add the rest if you like.