1st pass plugins for game rules

- added htmlmapping for a mapping between HTML and Game
- removed pick from board and humanclient, now in pickplugin
- created plugin.ts for a basic plugin framework
- plugins can be registered from the _setup code to make a number of
rules available
- each plugin provides functions for: createRule (returning a rule
structure), createPlugin (returning a plugin struture for clients) and
performCommand (for modifying the board)
- plugins are registered via a registerRule(rule: string, info:
PluginInfo), which adds this rule to the Board, and enacts the
associated plugins.