Fixed hardcoded tag filename prefix

posts_with_tags() invoked sed with a hardcoded `tag_&.html`.
Changed to `$prefix_tags&.html`.
don't invoke sed when no tag given

the bug was that sed was expecting file content on stdin when didn't receive
any filename, and script was "frozen" when someone tried to edit a post
without any tags.
suppress error messages and don't enter for loop when no tag files exist
Merge pull request #54 from Puuhinen/master

Fixed hardcoded tag filename prefix
Revert "Fixed hardcoded tag filename prefix"
Merge pull request #56 from cfenollosa/revert-54-master

Revert "Fixed hardcoded tag filename prefix"
Merge pull request #55 from Lex-2008/small-fixes/5

Fixes for blogs without tags
Manual fix for #54
Fixed #56
Sed error on mac
Sed error on mac
Sed error on mac
Sed error on mac
Sed error on mac