Add __getitem__ and __contains__ method to Config object

[djhoese]

These methods allow for bracket syntax access of config keys (ex. mypkg_config['a']) and in checks (ex. 'a' in mypkg_config). These should make it easier for dask to migrate away from using the config dictionary that should be considered internal use only.

As far as I can tell these should work in all of the expected cases since they are just wrappers around the more complicated get method. I'm not sure if there is a cleaner looking way to do the try/except inside contains without doing self[item].

@mrocklin @jhamman

[coveralls]

Pull Request Test Coverage Report for Build 21

• 34 of 34 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 93.849%

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Totals
Change from base Build 18:	0.9%
Covered Lines:	473
Relevant Lines:	504

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💛 - Coveralls

[codecov]

Codecov Report

Merging #2 into master will increase coverage by 0.85%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage   92.99%   93.84%   +0.85%     
==========================================
  Files           4        4              
  Lines         471      504      +33     
==========================================
+ Hits          438      473      +35     
+ Misses         33       31       -2

Impacted Files	Coverage Δ
donfig/config_obj.py	90.94% <100%> (+1.15%)	⬆️
donfig/tests/test_config.py	98.7% <100%> (+0.15%)	⬆️

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