use file basename by default

The github API doesn't seem to like file names with a '/' in it, like
you would get if you executed `gist /tmp/foo.txt` or
`gist subdir/bar.c`.  This yields a "422 Unprocessable Entity" error
which can be very confusing.  Using the file's basename eliminates this
problem.  This behavior can be disabled by passing `--no-basename` to
gist.