support headings between groups of options

[jclulow]

For complicated commands with several disjoint sets of options, it should be possible to visually group options together in the generated usage message. The grouping should allow for a heading string to be printed to separate the group from other options, and for that heading string to be (potentially) differently indented. e.g.

$ node some_program.js -h
ERROR: unknown option: "-h"
  first group:
    -f, --first-one
    -F, --first-two

  second group:
    -s, --second-one
    -S, --second-two

[trentm]

done with pull #6 published in dashdash@1.6.0