add instrumenter option parse

[ouhouhsami]

Based on issue #7 I add the possibility to give a specific istanbul instrument for browserify-istanbul.

[bassettsj]

+1 here.

[devongovett]

Just merged #11 which allows passing an instrumenterConfig option to configure Istanbul. Does that work for you?

[lukescott]

This is also needed for isparta, which is similar but uses 6to5. The instrumenter needs to be replaced as it "sub-classes" istanbul to transform es6 code and fix the line numbers.

The PR works, but has to be used like this:

{transform: [istanbul({
    ignore: ["node_modules/**/*.js", "test/**/.js"],
    instrumenter: new (require("isparta")).Instrumenter()
})]}

I made a comment above, which should allow it to be this instead:

{transform: [istanbul({
    ignore: ["node_modules/**/*.js", "test/**/.js"],
    instrumenter: require("isparta")
})]}

[devongovett]

Closing since #14 was merged.