Add the vimtutor shell script
This is the exact same vimtutor from the vim source repository:
https://vim.googlecode.com/hg/src/vimtutor
It works as is in MSYS, although there are a couple of warnings:
/bin/vimtutor: line 28: mktemp: command not found
/bin/vimtutor: line 28: tempfile: command not found
Since these utilities are not present.
However, the script has a contingency for that scenario, and manages to
create a temporary copy of the tutor file anyway, so all is well.
Signed-off-by: Curtis Rueden <ctrueden@wisc.edu>