Modified Jonah's behaviour

This updates Jonah's idiosyncratic engine in the following manners:
* All passages are now added to the bottom, regardless of where you
click the link.
* Clicking a link to a previously displayed passage will display a new
version of the passage at the bottom, instead of scrolling up to the old
version.
This is based on a patch I've authored based on code by Stormrose
(http://www.glorioustrainwrecks.com/node/5136) and results in more
intuitive and desirable stretchtext behaviour. I'm so confident that
authors would rather have this than Jonah's former TiddlyWiki-esque
behaviour, that I'm not even leaving any trace of the old behaviour
behind.
Variables in the link syntax

You can now use variables and expressions in the link syntax, such as
[[$door]] or [[Climb|"Climb" + $var]]. This works like so: if the
passage name is not found, it tries to parse it as a code parameter. If
this successfully returns a value, it is used as the passage name. This
is not a change I am making lightly, but I believe it to be the best
course of action.

* The syntax highlighter now tries to recognise code parameter links. It
currently just looks for $variables, or functions(), but is (much like
with external links) not comprehensive.
* Now, links in the image syntax are recognised as valid outgoing links
in the IDE.
* Altered the variable regexps to exclude all-digit variable names.
Merge branch 'jonah3'
Merge branch 'variable-links'