Improvement: Make docstrings out of old comments

Add docstrings for all the dispatch_* functions by refactoring their
comment headers into docstrings and formatting them to match a typical
manpage. The hope is that a future `help` command can access these to
provide a `help load` or `help alias` functionality trivially.
Feature: Modular help system

Rework the help command so that it is handled by a _dispatch command as
with all the others. `help` is able to list the implemented commands
(but not subcommands!) and access the documentation for any top level
command (which will include the subcommands!)