Add support for executing and viewing html docs

(In a horrible, hacky way -- can't recreate iframe, since I want
to share code between that and regular sandbox. Can't move iframe,
since that resets it. So I'm absolutely positioning it, with all
the problems that creates.)
Show line numbers when editing code
Improve line number info for error messages

(Slightly.)

Closes #7
Minor grammar fix. Should be "fewer" problems.