[MRG] Add support for a local config file

[betatim]

Fixes #21

[codecov-io]

Codecov Report

Merging #25 into master will increase coverage by 1.48%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   83.28%   84.77%   +1.48%     
==========================================
  Files          12       12              
  Lines         359      381      +22     
==========================================
+ Hits          299      323      +24     
+ Misses         60       58       -2

Impacted Files	Coverage Δ
osfclient/__main__.py	0% <0%> (ø)	⬆️
osfclient/cli.py	98.88% <100%> (+0.4%)	⬆️

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