implement summary with unlimited length

this commit fixes middleman/middleman#1074

If the summary_length is set to any value smaller than 0
and a summary_separtor is defined than the summary is
generated in case of
a) no separtor found in article: whole article (summary = article)
b) separator found in article: article until the occurence of the separator


This is my first pull request ever. So please be gentle with me. :grin: 

As soon as this got accepted I will add another pull request on middleman-guide to add a paragraph about it.