Simplify liquid documentation for clients

[epage]

Concerns

• Each tool might have its own flavor of liquid (set of filters, tags)
• A central place to document these could be confusing
• Keeping documentation away from code might make it out of date

[epage]

My proposal is to expand the filter/tag API so that they can document their behavior. The top-level Parser could either dump the results or let users iterate on it, unsure which.

Then clients of liquid could have a command line flag to print out the documentation.