Expand configurable documentation

Motivates the need for @configurable and tries to explain when you might want it. Preview available here.

The HLT2 line tutorial now links to this explanation, and clarifies the conventions of cut values in arguments. I've set this to close #54 (closed). Given we have a few hundred lines following this convention I don't want to spend more time debating it, so we'll see how it works it.

@rmatev would you mind reviewing the Tonic docs?