Node ES6 support

[megawac]

Hey, if you give me some pointers of where to start (I only took a glance would it be lib/cli?), I'll add support for this. It'd be sick if we could get es6 support for node testing through 6to5.

Works great for testing browsers with ES6 files already, having a bit of trouble getting it set up for node. I figure we can just add an es6 flag and use the 6to5 cli instead of node (http://6to5.org/docs/usage/cli/#6to5-node)

# Working browserify method
$ prova test/**.js -q -b -t 6to5ify -l firefox -e

# Hypothetical node method
$ prova test/**.js -es6

[megawac]

Got this working locally after applying azer/flat-glob#3 using the require hook from 6to5 in cli for importing, and a couple other patches.

/cc @azer

[gmaclennan]

I am having trouble with this. I can get the tests running in node using babel-tape-runner but when I run with -b I get this error:

    15.         return process.nextTick(function () {
    16.             cb(new Error('path must be a string'));
    17.         });
                       ^
    Error: path must be a string

Running the tests directly with babel-node gives the error:

(function (exports, require, module, __filename, __dirname) { import test from 'prova';
                                                              ^^^^^^

SyntaxError: Unexpected reserved word

[azer]

Sorry about not taking care of this Issue & PR. I'll look at this tonight