Check identifier syntax according to the standard

[yucca42]

Currently, JSHint seems to enforce the old identifier syntax, which allows Ascii characters only. For example, the statement
var état;
causes the error message "Unexpected 'é'." Worse still, this aborts the checking process: "Stopping, unable to continue."

However, the ECMAScript standard as well as implementations have allowed a much more liberal syntax (surely allowing "état") for several years. Please implement identifier syntax as defined in ECMA 262 clause 7.6.

Perhaps an informative message could be issued upon encountering the first non-Ascii character in identifier, just in case it's accidental and not intentional.

[valueof]

Marking as accepted. Thanks!

[goatslacker]

Similar #301

[valueof]

Closing in favor of #301 (more discussion there).