Passage warnings

* In addition to the aforementioned passage editor helpers, I've now
added passage-close warnings. These display whenever presumed errors are
detected in passage code whenever said passage is closed. Warnings are
defined in Header.py and can be altered on a
story-format-by-story-format basis.
* This is, of course, controlled by a Twine preference.
* Based on feedback, I've changed the prohibition of = in <<if>> from a
run-time error to a passage-close warning.
* The other warning I've added, for now, is the presence of unquoted
"<script>" tags in script passages, due to people improperly pasting
HTML into them.
* Changed the remaining functions in header.py to camelCase for internal
consistency.