Make it possible to specify display of stats in a particular 'container' by separating category and container with a period.

e.g. "show stats server.network"
I failed to realize this had already been implemented without the period in the show stats command (as the command help had not been updated).
However, I would prefer the . approach as it will allow specifying multiple stats, easier wildcarding, etc.
This commit also prevents any stat from having a period in its short name.