Add special passage names and tags for SugarCube

This is a temporary solution; it would be better to make the header-
specific Python module register these.
Removed unused method TiddlyWiki.tryGetting()
Ignore text before first passage declaration in Twee source

Example:
===
Some Twee sources...
...have text before first declaration.
:: Start
Once upon a time...
===

Before this commit, a passage titled "Some Twee sources..." would be
created from this example. After this commit, the text before the
first passage is ignored.