implement NSButton extension for OSX

[jzau]

I am not sure if there's a way to combine UIButton with NSButton. So I didn't change UIButton+extension, for now, I just create a new file to implement NSButton as it doesn't have backgroundImage property but alternateImage instead.

[codecov-io]

Current coverage is 82.64%

Merging #287 into master will increase coverage by +0.50% as of e1b2537

@@            master    #287   diff @@
======================================
  Files           12      13     +1
  Stmts         1322    1406    +84
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit           1086    1162    +76
  Partial          0       0       
- Missed         236     244     +8

Review entire Coverage Diff as of e1b2537

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[onevcat]

@JackyZh Sorry for the late reply!

Thanks for the pull request. It seems cool at first glance. I will take a look deeper later. Since the APIs of UIButton and NSButton are quite different, I guess it should be no problem to keep them separated. It will not be hard to combine them into one too, if we'd like to do so later.

Cheers.