fix: added missing 'newsblur' prefix to config value name.

The missing 'newsblur' prefix to the 'newsblur-min-items' config value
would initialize the 'min-items' variable with an arbitrary, possibly big
value and let newsbeuter to endlessly fetch feeds.
Merge pull request #114 from RobinKrom/master

fix: added missing 'newsblur' prefix to config value name.