getitem support for standard_name

[dcherian]

Closes #37
Closes #4

[codecov-commenter]

Codecov Report

Merging #39 into master will increase coverage by 1.50%.
The diff coverage is 98.37%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   89.84%   91.35%   +1.50%     
==========================================
  Files           6        7       +1     
  Lines         384      474      +90     
==========================================
+ Hits          345      433      +88     
- Misses         39       41       +2     

Flag	Coverage Δ
#unittests	91.35% <98.37%> (+1.50%)	⬆️

Impacted Files	Coverage Δ
cf_xarray/accessor.py	93.43% <97.61%> (+1.16%)	⬆️
cf_xarray/tests/datasets.py	100.00% <100.00%> (ø)
cf_xarray/tests/test_accessor.py	100.00% <100.00%> (ø)

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