Remove --symbols option and make it default

[ansasaki]

This make '--symbols' default for the 'update' command. This way the strategy option is no longer required and will be assumed as the same as the current '--symbols'. This means if '--add' and '--remove' are not provided, the provided symbols will be compared with the existing symbols to create the set of added and removed symbols.

Also automatically generates usage description documentation.

[codecov-io]

Codecov Report

Merging #40 into master will increase coverage by 0.12%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   95.91%   96.03%   +0.12%     
==========================================
  Files           3        3              
  Lines         734      732       -2     
  Branches      177      176       -1     
==========================================
- Hits          704      703       -1     
  Misses          6        6              
+ Partials       24       23       -1

Impacted Files	Coverage Δ
src/smap/symver.py	95.97% <100%> (+0.12%)	:arrow_up:

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[coveralls]

Pull Request Test Coverage Report for Build 85

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 97.419%

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Totals
Change from base Build 83:	0.08%
Covered Lines:	726
Relevant Lines:	732

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💛 - Coveralls