Add exception for escaped keywords

[ghost]

Same as: jquery/esprima#1612

[marijnh]

We're currently interpreting that part of the standard to mean that such words shouldn't be treated as keywords, but it seems everybody else is taking it to mean an error should be raised if the resulting unescaped word is a keyword.

Attached patch adjusts this.

[ghost]

Sorry! I didn't know... Thx for the quick commit, anyways