Rename long option for -s

[blueyed]

It actually does not swallow output, so --no-capture is a better name.

Based on that pytest has --capture=no for -s.
Maybe something positive like --keep-output or --display-output would be better?

[codecov-io]

Codecov Report

Merging #107 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #107   +/-   ##
=======================================
  Coverage   89.61%   89.61%           
=======================================
  Files           3        3           
  Lines         905      905           
=======================================
  Hits          811      811           
  Misses         94       94

Impacted Files	Coverage Δ
mutmut/__main__.py	84.59% <100%> (ø)	⬆️

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[boxed]

Aha. Yea then I think we should follow pytest.

[l0b0]

@blueyed Not sure if you're aware, but this needs a rebase.