Fix bracketing for non-required positionals in usage

[alexflint]

In the usage string we now put square brackets around non-required positional arguments, and nest them all together like this:

Usage: myprog REQUIRED1 REQUIRED2
Usage: myprog REQUIRED1 REQUIRED2 [OPTIONAL1 [OPTIONAL2]]
Usage: myprog REQUIRED1 REQUIRED2 REPEATED [REPEATED ...]
Usage: myprog REQUIRED1 REQUIRED2 [REPEATEDOPTIONAL [REPEATEDOPTIONAL ...]]
Usage: myprog REQUIRED1 REQUIRED2 [OPTIONAL1 [REPEATEDOPTIONAL [REPEATEDOPTIONAL ...]]]

Fixes #174

[codecov-commenter]

Codecov Report

Merging #175 (5fb236a) into master (d370610) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #175   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          682       683    +1     
=========================================
+ Hits           682       683    +1     

Impacted Files	Coverage Δ
usage.go	100.00% <100.00%> (ø)

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