Feature: Add a directive system

[enderger]

Adding directives

Currently, LMT provides a great way to weave code from most flavors of Markdown. However, the lack of a directive system makes it dificult to provide flexibility. I propose a system of directives, which can be used for slight alterations of the program's behavior.

Syntax

I propose that directives take the following syntax, which retains compatibility with most forms of Markdown:

<!--@<name> <paramaters>-->

This syntax uses HTML comments to hide these directives in the output, while still remaining distinct from other comments

Suggested Directives

A few directives sorely missing are:

• @namespace <name>, which causes everything up to the next namespace directive to be handled separately. This provides a concrete mechanism for avoiding accidentally overwriting code blocks.
• @defaultAppend <bool>, which determines whether to append by default (see Consider making += the default #14).
• @requireUndefined <name>, which requires that a macro is not defined outside of this file (again, preventing collisions).