Handle explicit empty placeholders

[alexflint]

When the user explicitly sets the placeholder to empty, we change the usage text so that there aren't extraneous spaces:

var args struct {
    Something `arg:"-s" placeholder:"" help:"some help for something"`
}

...
Options:
  -s, --something    some help for something
...

Previous this would have printed usage text with some extraneous spaces: -s , --something

Closes #241

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.17%. Comparing base (5ec29ce) to head (be792f1).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
- Coverage   99.19%   99.17%   -0.02%     
==========================================
  Files           5        5              
  Lines         745      731      -14     
==========================================
- Hits          739      725      -14     
  Misses          3        3              
  Partials        3        3              

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