Add support for some underscored harmony flags as well

[dominicbarnes]

This adds support for both hyphen and underscored flags. (at least a few of them)

Here's my reasoning: I'm trying to run CI tests against several versions of node. (0.10, 0.11, 0.12 and iojs to be precise) I'm using generators in my code, so:

• iojs breaks if I add --harmony-generators
• 0.11 and 0.12 require --harmony-generators
• 0.10 doesn't support generators at all, but I'm using gnode as a last-resort fallback

Rather than doing fancy "version sniffing", I've opted to use a clever shell script to determine if the node executable supports the generators flag, and using that to make my determination:

$ node --v8-options | grep generators | cut -d ' ' -f 3
--harmony_generators

Unfortunately, passing this value directly to mocha breaks, since v8-options uses the underscores instead of the hyphens. (but the node exe definitely acknowledges both in all cases I've tested)

tl;dr: I know it's ugly, but I think this pattern will be useful to others while transitioning between node 0.10/11/12+ and iojs. Thanks :)

[jbnicolai]

Let's hope we can remove this in the near future, with iojs and node merging. But for now: looks like a good fix, and thanks for the explanation!