allows pelican-themes symlink command to use relative paths not beginnin...

...g with a dot

`pelican-themes -s <dir-name>` creates a broken symlink when the `<dir-name>` is a relative path not beginning with a dot, i.e. `vendor/themes/clear-theme`. Afterwards the theme does not show up with `pelican-themes -l`. Notice that the `-i` command *does* allow for this path style, since it copies the entire directory for the new theme to the themes archive directory that pelican references. 

This change makes sure that the symlink command makes a correct symlink. A test file has also been added for `pelican-themes`.