Add ability to override default 'example' namespace.

[emmanvg]

Related to #32

Added --use-namespace option.

[codecov-io]

Codecov Report

Merging #33 into master will increase coverage by 0.15%. The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   64.63%   64.78%   +0.15%     
==========================================
  Files          16       17       +1     
  Lines        5033     5055      +22     
==========================================
+ Hits         3253     3275      +22     
  Misses       1780     1780

Impacted Files	Coverage Δ
stix2slider/cli.py	32.43% <0%> (-0.91%)	:arrow_down:
stix2slider/convert_stix.py	66% <100%> (+0.24%)	:arrow_up:
stix2slider/utils.py	89.47% <100%> (+2.8%)	:arrow_up:
stix2slider/test/test_namespace.py	100% <100%> (ø)
stix2slider/test/test_idioms.py	77.67% <100%> (-0.59%)	:arrow_down:
stix2slider/options.py	69.93% <50%> (+1.06%)	:arrow_up:

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[rpiazza]

@emmanvg @gtback: Hold off on merging this - I made some changes too that haven't been pushed, which I think conflict with what was done here.

[rpiazza]

@emmanvg: your changes subsume mine, so we should just merge this in and I will delete my branch. One thing - can you add --use-namespace to command-line.rst, then it will be ready for merging.

[emmanvg]

@rpiazza @gtback, it should be good to go now. I have added a test as well.