Added non-href option to summarize filter

I came across an issue with the summarize filter while creating a theme.

Sometimes the continue link is not needed directly after the summarized version of an article but somewhere else. In the current version the must be a ``%s`` present when using the standard filter. Just using a simple string results in an exception. This is problematic when someone likes to just append plain text like ``&hellip`` without any link.

Therefore I simply added a new method to centralize the link-insert action and checked whether or not a ``%s`` is present in the ``options['link']`` parameter.

This request also adds a sentence to the documentation to explain the now optional ``%s`` within the ``SUMMARIZE_LINK`` configuration parameter.

Sorry for the typo in the commit message - hope this is isn't too bad :smile: