Added bookmark()

* The bookmark() function lets the author create Bookmark links in their
passages by simply doing [[Bookmark|bookmark()]]. Hopefully it works
out.
* Fixed bugs in engine.js relating to bullet-point syntax, and reading
the Obfuscate StorySetting.
* Now, HTML files created via Test Play From Here won't override any
bookmark hashes applied to the end (for testing purposes etc.)
* Now, the StorySettings passage's text is stripped of the old
ObfuscateKey value, as well as old comment lines.
Syntax bugfixes

* A close-bracket in a link's expression, such as [[ $arr[0] ]], should
no longer be considered invalid.
* A double-right-chevron in a macro's argument, such as <<print ">>" >>,
should no longer terminate the macro tag prematurely.