em: Allow escaped underscores in em again

[ariabuckles]

This was a regression in 0.3.4

See issue #641

Test plan:

In node, run

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'

[ariabuckles]

Update: forgot to build the minified version, and forgot to do [\s\S] instead of _ after the backslash :/. I updated the commit.

[joshbruce]

See #1010

Closing due to merge conflicts, stale, and missing tests.