Feature: Make kube-state-metrics a library

[lilic]

Is this a BUG REPORT or FEATURE REQUEST?:
/kind feature

What:
This is a follow up from #575. Some functions were exposed in that PR, but would like to continue the disucsion to make more of a usuable library.

How:

pkg
├── collector // functions needed to create a collector, this would be used in the builder pkg and externally 
├── internal
│   └── builder // content of the current upstream resource collectors which would consume the collector pkg
├── metrics // what is currently there but also move the utils/helper functions from collectors. Also the metricHandler interface as that would be useful to expose and reuse elsewhere.

cc @mxinden @brancz

[lilic]

@mxinden @brancz Any thoughts on the above? I would be willing to do the work on this. :) Thanks!

[brancz]

Moving anything that is not lib or main to an internal package sounds like a good idea. I think I would like to hold off on rearranging before the final v1.5.0 release (which we should be cutting rather soon). How does that sound?

[mxinden]

With v1.5.0 cut, I am happy to review the refactoring suggested above. Feel free to ping me @lilic on the PR.

[mxinden]

//CC @wanghaoran1988 for the work on https://github.com/wanghaoran1988/openshift-state-metrics