Add custom adapter option

[jerryorta-dev]

I want to keep all benefits that karma-jspm offers, but would like the opportunity to develop a little lower level by writing my own adapters as necessary.

For angular2 with typescript tests I'm finding it necessary to write a custom adapter.

I have a PR ready, and have a sample implementation at https://github.com/UIUXEngineering/angular2-seed-jspm/blob/master/src/client/jspm.karma.ng2.adapter.js.

This is a non-breaking change in that if no custom adapter is provided, the default adapter is used.

This may also help with #162

[RIAstar]

Unfortunately, the PR won't help with #162 because that bug causes none of the files to be loaded at all. That would include the custom adapter.