simplify link/comments selectors

was there a reason for .even and .odd in those selectors?
filter links/comments by visible

Although other extensions (read: RES) hide some posts per user filters, F7U12-Link-Opener still opens links/comments for these posts.  This change makes this extension ignore hidden posts.
Merge pull request #1 from andytuba/master

Ignore hidden posts when opening all links/comments