Deterministically order plugins, create cross-plugin call mechanism

Added a PluginManager.Call(function_name, ...) that allows plugins to
call functions by name.  The plugin manager searches all plugins for
functions matching the name and signature of the arguments, then calls
each in turn.  It in effect creates a simple message-passing system
where callers and callees are anonymous.

Added DeleteSession() to Session, and modified Vimish's command-line to
use the above Call system for non-internal functions.