Allow script path to include spaces

[JonathanHolvey]

Without this, if the script path contains spaces, the _base16 function will fail to source the script and create the theme symlink:

bash: [: /home/Jonathan: binary operator expected
ln: target '/home/Jonathan Holvey/.base16_theme': Not a directory

This is using Git Bash on Windows, where the home directory path may include spaces, however this could also conceivably happen if base16-shell is installed in a non-standard location on a UNIX-like OS.