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 - 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.
I have a feeling this could be a breaking change for those who build their pelican site locally and then copy to another machine/location to serve.