Adding support for getter/setter descriptors

[osi-oswald]

One more try :-)

Currently only descriptors with writable/value pairs are supported. This PR will add support for descriptors with getter/setter pairs.

Since these descriptor were never supported, this change will not add any regression and only add missing functionality.

I hope this upgrade makes it into the repo. All the best :-)

[osi-oswald]

Btw this also solves my previous issue #74, just in a different way.

Since descriptor.initializer.call(context) is called in any case, it can be used as a "hook into the initialization". Problem solved!

[osi-oswald]

Await, will add some tests

[loganfsmyth]

I still don't feel willing to make changes here. There are updates to decorators coming in the future for Babel 7, and I don't generally view this plugin as something we'll maintain once that happens. This plugin essentially existed to bridge the gap during Babel 6's lifetime.