update compatibility ranges (17-25)

Bump min-version to 17 ESR due to nsIParserUtils, CSS unprefixing and
Task.jsm/Promise.jsm availability
remove innerHTML from firstrun.xhtml
replace innerHTML with using parserUtils

This requires at least Firefox 14. In-item scripts are disabled, in-item
styles and external embeds are still enabled (may be disabled later).

Fixes #48 by disabling the in-item JS. However, #47 is likely not fixed
due to autoplay in <audio> and <video>. May lead to performance changes.
Affects #9 as relative links are now disabled completely.
use feed URL as base for parsing

Fixes #9 resolving relative URLs correctly
unprefix CSS properties available by Fx17
show star when hovering any part of the entry
if no unread with a tag, hide the count
increment version to 1.1.0