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)
> `string` can be `String.fromCharCode(0, 1, 2, ..., 0x10FFFF)`. Or some representative subset of that.

While it’s clear what you meant with that comment, note that [`String.fromCharCode()` only works the way you’d expect for code points up to U+FFFF](http://mathiasbynens.be/notes/javascript-unicode#code-point-to-symbol). In other words, avoid things like `String.fromCharCode(0x10FFFF)`.