Add "." command in instruction sheet

This is a no-op command that can be used to comment out an instruction
line without needing to remove it or indent it, causing it to be
included in the previous command's message.

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge branch 'dot-instruction'

New instruction to temporarily ignore an entry in the instruction sheet.

* dot-instruction:
  Add "." command in instruction sheet