Add TerminalView.

* Add hterm dependency.
* Add views/terminal_view.js, which executes a command of your
  choice from the axiom filesystem in an hterm instance.
* Add exe/readline.js, run it by default for now.
Add environment module.

* Add a module to maintain global references, so far only for the
  global module manager.
Convert readline to a promise based execute callback.
Initial add of string utilitiy functions.
Initial commit of wash and ls.

* Commit wash and ls executables.  They're somewhat working but not
  completely.  Builtins are not dispatching properly yet and need
  to be debugged.
Commit hterm into third_party for now.

Once hterm has a working npm package we can remove this.