twee: If a directory is specified as a source, recursively scan it

Files with the ".tw" and ".twee" (all lower case) file extensions are
considered Twee sources for this purpose.
Merge pull request #139 from mthuurne/twee_dirscan

twee: If a directory is specified as a source, recursively scan it
Header-supplied passage widget colours

* Header.py now has a passageTitleColor method which can provide colours
for passage widgets' title bars. It takes a Tiddler object as its
argument.
* The colours for stylesheets, scripts, info passages, and others whose
meaning is only really determined by the runtime engine, are now defined
in the base header.
* Tweelexer: renamed badLinkStyle to linkStyle and made it a TweeLexer
static method.
* The Twine app's version number is now in the window title.
* Fixed a bug where passages with external links but no internal links
would not be given the "terminus passage" colour.