Allow fill-value on conversion from NumPy.

[hameerabbasi]

Allows specifying a fill-value when converting from a NumPy array.

[codecov]

Codecov Report

Merging #180 into master will increase coverage by 0.39%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #180      +/-   ##
=========================================
+ Coverage   96.91%   97.3%   +0.39%     
=========================================
  Files          11      11              
  Lines        1295    1301       +6     
=========================================
+ Hits         1255    1266      +11     
+ Misses         40      35       -5

Impacted Files	Coverage Δ
sparse/coo/core.py	96.78% <100%> (+0.04%)	⬆️
sparse/sparse_array.py	96.66% <100%> (+3.56%)	⬆️
sparse/compatibility.py	83.33% <0%> (+33.33%)	⬆️

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