Add nodata, dtype options

[virginiayung]

Resolves: https://github.com/mapbox/rio-pansharpen/issues/17 Status: WIP, NOT ready to merge

[codecov-io]

Current coverage is 96.85% (diff: 100%)

Merging #22 into master will increase coverage by 0.07%

@@             master        #22   diff @@
==========================================
  Files             5          5          
  Lines           124        127     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            120        123     +3   
  Misses            4          4          
  Partials          0          0          

Powered by Codecov. Last update 28e2446...fd7a256