em: Allow escaped underscores in em again

This was a regression in 0.3.4

See issue markedjs#641

Test plan:

In node, run

```javascript
var marked = require("./lib/marked.js");
marked.parse("_ABC\\_DEF_")
> '<p><em>ABC_DEF</em></p>\n'
```

Previously, this would give '<p>_ABC_DEF_</p>\n'

lib/marked.js

@@ -456,7 +456,7 @@ var inline = {
   reflink: /^!?\[(inside)\]\s*\[([^\]]*)\]/,
   nolink: /^!?\[((?:\[[^\]]*\]|[^\[\]])*)\]/,
   strong: /^__([\s\S]+?)__(?!_)|^\*\*([\s\S]+?)\*\*(?!\*)/,
-  em: /^\b_((?:[^_]|__)+?)_\b|^\*((?:\*\*|[\s\S])+?)\*(?!\*)/,
+  em: /^\b_((?:[^\\_]|\\_|__)+?)_\b|^\*((?:\*\*|[\s\S])+?)\*(?!\*)/,
   code: /^(`+)\s*([\s\S]*?[^`])\s*\1(?!`)/,
   br: /^ {2,}\n(?!\s*$)/,
   del: noop,