add default value for arg --no-colour, --no-prefix

[laobaox]

without default value for arg --no-prefix, you can not use:
honcho --no-prefix start
the --no-prefix dost not work, becase overide by sub command arg

you can only use:
honcho start --no-prefix

so, to add default value to support:
honcho --no-prefix --no-colour start

[codecov-commenter]

Codecov Report

Merging #223 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   49.16%   49.24%   +0.08%     
==========================================
  Files          11       12       +1     
  Lines         598      599       +1     
  Branches       88       88              
==========================================
+ Hits          294      295       +1     
  Misses        293      293              
  Partials       11       11              

Impacted Files	Coverage Δ
honcho/command.py	0.00% <ø> (ø)
honcho/__init__.py	100.00% <0.00%> (ø)

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[nickstenning]

Thank you for fixing this!