Adding an optional -b BRANCH argument to build_examples.py to enable
comparing to and restoring from any branch or commit. That will help
when e.g. reversing an unfortunate commit of generated files.

This solves part 2 of issue wireviz#167.

When used to "git diff", it is easy to write "build_examples.py diff"
instead of "build_examples.py compare" by a mistake. Therefore,
"diff" is now an alias to "compare" so both commands will work.

This solves part 3 of issue wireviz#167.

It seems, that the current implementation ignores --generate-bom,
and that parser.add_argument() code line is therefore commented
out until it gets implemented to avoid any confusion.

This solves part 1 of issue wireviz#167.

Add -V command line option (and --version as an alias) to
both wireviz.py and build_examples.py that show version number.

Move the version number from setup.py into __init__.py to access
the same version number specification from all files needing it.

This solves part 4 of issue wireviz#167.