@TODO Nov 13th, 2020 Ben Culkin :InvokeDirective
This is more a 'mad-science' kind of thing, but creating a directive that allows
people to invoke invokable items passed as format arguments could be interesting.
	The key-name for this directive would probably be `I, and there are a number
	of invokable types that could be interesting
	* Supplier<String> to just insert the resulting string.
	* Function<Object, String> to convert a format item to a string.

@TODO Nov 13th, 2020 Ben Culkin :ConfigDirectives
Add a way to configure user-based directives from reading a config file, which
allows you to bind directives to various characters and customize their behavior.
One thing that will need to be involved is that all of the directives should
probably have their interfaces tweaked to allow you to provide a custom form of
them. One could argue this is the sort of thing that macros could do, but I can
see no compelling reason not to implement both features

@TODO Nov 14th, 2020 Ben Culkin :OnceDirective
Implement a directive (`O probably) that will only be executed once per given
invocation of 'format', or maybe one for 'once per enclosing directive'. Could
be useful, especially once we have more stateful directives.