add xref for subcommands

[crotwell]

subcommands get separate html files, nice if html generated from asciidoc could link them.

Added an xref for each subcommand and updated tests. I believe an xref like this doesn't hurt anything, and helps with linking is possible (html and pdf). That said, I know just enough asciidoc to be dangerous, so...

[codecov]

Codecov Report

Merging #1340 (b328ed4) into master (0c5a971) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1340   +/-   ##
=========================================
  Coverage     93.76%   93.76%           
  Complexity      474      474           
=========================================
  Files             2        2           
  Lines          6961     6961           
  Branches       1869     1869           
=========================================
  Hits           6527     6527           
  Misses          147      147           
  Partials        287      287           

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[remkop]

Merged.
Thank you for the contribution! I really like this, and suspect there are many other parts of the document that can be linked this way. For example:

• provide a link to the parent command (maybe from the synopsis)
• provide links to the options from the synopsis
• ... other ideas?

Also related to #1227 in the sense of improving usability of the generated manpage docs.