Extended metadata handling.

[ivan-shaban]

Hi Shaun!
For now swiftsuspenders extract some meta-information by describeType methods, which are works great but quite expensive by performance. Also, often, we need to handle some custom data for classes that maybe already parsed by swiftsuspenders, and in that case we should call describeType methods one more time, as i mentioned early that can affect a little performance, what i wanna to avoid. So my decision is to add support external meta-information processors and here i wanna ask you, what do you think about it? Mb you encountered it early and have any plans how to implement it in best way?