allows pelican-themes symlink command to use relative paths

`pelican-themes -s <dir-name>` creates a broken symlink when the `<dir-name>` is a relative path - i.e. `vendor/themes/clear-theme`. Afterwards the theme does not show up with `pelican-themes -l`. Notice that the `-i` or `install` command *does* allow for relative paths.

This change makes sure that the symlink command makes a correct symlink. A test file has also been added for `pelican-themes`.