Remove byte order markers when reading input files

By using the "utf_8_sig" encoding, byte order markers (if any) are
removed from input files as they are read.
Added updated version of "twee" command line compiler

Having Twee in a separate package doesn't really work, since various
components such as TiddlyWiki and Story Formats do not stay in sync
with Twine. It is better to share the same code and data between the
command line compiler and the GUI.

Differences compared to the old "twee":
- dropped options: plugins, RSS generation
- added option: override start passage
- multiple merge files allowed
- "sugarcane" as default target
- all error messages are written to stderr
Deduplicate version string

Create a dedicated module for the version string, so it exists in a
single place. This simplifies version bumps and avoids mistakes.