"Option as Meta" for OS X

[pzich]

One commonly used setting in Terminal on OS X is to use the option key as the meta key (see below).

This should be easy to add as an option to the Terminal object by creating a flag like optionAsMeta and changing (!isMac && ev.altKey) || (isMac && ev.metaKey) in Terminal.prototype.keyDown to (!isMac && ev.altKey) || (isMac && (this.optionAsMeta ? ev.altKey : ev.metaKey)).

Let me know if you'd prefer this as a pull request.