The recent change that introduced python3 support also made it the default
python version used for Gundo. This caused a problem with certain versions of
vim where upon toggling the Gundo window, we would be greeted with an error
message: "This Vim cannot execute :py3 after using :python"
This commit adds a g:gundo_prefer_python3 variable (default 0) that can be used
to set a preference for python3. By default, python2 will be used.
This also required moving the variable initialization code to the top, since
just evaluating `has('python3')` seems to trigger the vim error.
