Add a new comman-line entrypoint symver-smap

[ansasaki]

This is useful specially for distributions where the name 'smap' conflicts with existing applications.

This also makes the name of the command-line application to be used in the output files instead of the hard-coded name 'smap'. The tests have been updated accordingly.

[coveralls]

Pull Request Test Coverage Report for Build 152

• 12 of 12 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 98.489%

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Totals
Change from base Build 150:	0.02%
Covered Lines:	740
Relevant Lines:	740

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #73 into master will increase coverage by 0.03%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   97.53%   97.56%   +0.03%     
==========================================
  Files           4        4              
  Lines         729      740      +11     
  Branches      187      190       +3     
==========================================
+ Hits          711      722      +11     
  Partials       18       18

Impacted Files	Coverage Δ
src/smap/symver.py	97.52% <100%> (+0.03%)	:arrow_up:
src/smap/main.py	100% <100%> (ø)	:arrow_up:

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