support pandas nullable integer string aliases

[cosmicBboy]

This PR adds support for pandas_dtype accepting string aliases for the pandas nullable integer array data type.

[codecov-commenter]

Codecov Report

Merging #244 into master will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   96.33%   96.20%   -0.14%     
==========================================
  Files          15       15              
  Lines        1284     1395     +111     
==========================================
+ Hits         1237     1342     +105     
- Misses         47       53       +6     

Impacted Files	Coverage Δ
pandera/schemas.py	96.15% <ø> (ø)
pandera/__init__.py	100.00% <100.00%> (ø)
pandera/dtypes.py	94.11% <100.00%> (+1.13%)	⬆️
pandera/schema_statistics.py	100.00% <0.00%> (ø)
pandera/io.py	90.47% <0.00%> (+0.19%)	⬆️

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