Add SugarCube header and make it the default.

Added the SugarCube header from http://www.motoslave.net/sugarcube/, which is
an enhanced fork of the Sugarcane header included with stock twine. There have
been a number of fixes to various functions as well as additional macros that
make authoring twine stories easier. Used with permission of the author.
Fix toRSS function as well as add SugarCube specific link handling

SugarCube has a number of additional link types to passages that need to be
supported in the link handler. These include bind, link, back, and return
macros. Also the toRSS functionality of the library was fixed, though will
likely be removed at a later date, due to being unused within any of the twine
code.
Add debug tag which prevents the stripping of newline characters.

A debug tag was added to the library which basically prevents it from stripping
all the newline characters from a passage, converting them to \n, and
compressing the entire content of the story to reside upon 1 line. This was
primarily to help aid in debugging any script passages.