Add support for App Extension target

[tripleCC]

Inspired by SDWebImage, using runtime to judge whether sharedApplication() is available in current target #269 .
It also can be done with the compile flags

[codecov-io]

Current coverage is 81.92%

Merging #290 into master will decrease coverage by -0.22% as of 710dc7e

@@            master    #290   diff @@
======================================
  Files           12      12       
  Stmts         1322    1328     +6
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit           1086    1088     +2
  Partial          0       0       
- Missed         236     240     +4

Review entire Coverage Diff as of 710dc7e

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[onevcat]

Brilliant! Thanks for this.