* Example on _e, but other commands are probably affected
* When completing, throw CompletionRequest("_error", <message>) to have
a reasonable output and not mess up readline
* Avoid early returns (or if a command really cannot be completed,
adjust the ShellCommand declaration to not run the command)
* Less CommandError in the parser, these would throw during
autocompletion and annoy everyone
* Allow options with empty value so that we don't throw when
autocompleting after typing eg. 'vspace='
* Change option syntax to <name>=<value>, with mandatory "=". This is
still not great, but manageable.
* Remove the Session::require_vspace() function, whose named overload
was misused (to check for the existence of a named space), and the
associated exceptions. Explicit error flow is better error flow.
* Change the autocompletion system to generate all options in a vector
of strings and then iterate, rather than using generators.
* Add completion for symbols.
Lephenixnoir