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_((?:[^\\_]|\\[\s\S]|__)+?)_\b|^\*((?:\*\*|[\s\S])+?)\*(?!\*)/,
  code: /^(`+)\s*([\s\S]*?[^`])\s*\1(?!`)/,
  br: /^ {2,}\n(?!\s*$)/,
  del: noop,