Support case-insensitive Bag-Info tags (Issue #21)

[tdilauro]

Fixes/implements #21 (case-insensitive Bag-Info tags).

• Adds ignore_baginfo_tag_case property to Profile class;
• Adds -i and --ignore_baginfo_tag_case flags used to set the associated Profile property;
• Appends help text to some error messages to indicate that an "ignore case" option is available;
• Adds new tests.

[codecov[bot]]

Codecov Report

Merging #22 into master will increase coverage by 1.88%. The diff coverage is 91.93%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   80.73%   82.61%   +1.88%     
==========================================
  Files           2        2              
  Lines         436      489      +53     
==========================================
+ Hits          352      404      +52     
- Misses         84       85       +1     

Impacted Files	Coverage Δ
bagit_profile.py	71.02% <80.95%> (+1.28%)	:arrow_up:
test.py	98.54% <97.56%> (-0.25%)	:arrow_down:

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[tdilauro]

No problem.