Allow definition of "helper functions"

[agnoster]

There should be a way to define and call helper functions like "log in" or "today's date", etc.

However, we need to define:

Where should the functions be defined?

We could search some "base" directory for a literapi.js file, or similar. Or we could have a -I "include" flag for literapi that points to a file to be included.

How should the functions be called from the LiterAPI doc?

Simply having some special syntax seems like it would defeat the human-readability of LiterAPI, and obscure the meaning. Since there are no comments in Markdown, there's no good way to embed a function call (unless added to the opening fence of a code block, but that feels... hacky).

We could do some kind of text-matching a la cucumber, so something like

> NB: We assume a logged-in user. See [[Authentication]] for more details.

Would be matched by

literapi.match(/^We assume a logged-in-user./, function(){ ... })

(In this case, assuming "NB" is a well-defined scope prefix so it's easy to find all LiterAPI directives in a document.)