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.
The existing pelican-themes -s <relpath> command is broken. If the linked theme is specified in the pelican configuration file and a build is attempted, the pelican build command will error out with a "theme not found" error message. It is not possible that people are building locally with current pelican-themes --symlink behavior.