present: make code in slides editable only with -edit

Also added a suite of tests and fixed a minor bug that
caused a panic when the .code command specified HL with
no text after.

Fixes golang/go#17379.

Change-Id: I3c246523c3d4010bf76a467ee648475255090e1b
Reviewed-on: https://go-review.googlesource.com/30691
Reviewed-by: Rob Pike <r@golang.org>
3 files changed