v6.0.4

Mainly a bug-fix release.
The code base was also cleaned up in many places, which should lead to a bunch of performance improvements.

• Add support for 'age' post-filter (issue #1354)
• New github issue templates
• Move async logic from Site to client classes (issue #1342)
• Log errno when failing to set file mod time on unix
• Fix crash when adding empty header in source settings
• Fix page count parsing for Danbooru since switch to Font Awesome (issue #1331)
• Save 'search' when downloading single images (issue #1322)
• Allow to set an user and password to proxy (issue #1350)
• Force thumbnail extension to JPG on GB 2.0 XML (issue #1348)
• Don't change wiki from non-current tabs (issue #1347)
• Make wiki text selectable (issue #1347)
• Fix wiki link parsing (issue #1347)
• Fix crash on startup when not restoring tabs
• Set image count to tag count if coherent with page count (issue #1336)
• Multiple performance and memory usage improvements
• Fix Danbooru parsing with '+' and '-' tag links (issue #1331)
• Use 'max image count' in display and batch downloads (issue #1331)
• Calculate 'max image count' when parsing page tags (issue #1331)
• Don't trust calculated count from tags for image count (issue #1329)
• Add 'grabber' meta-tag for post-filtering (issue #1325)
• Fix JSON API url for Sankaku (issue #1250)
• Fix Zerochan always loading sample images (issue #1317)
• Fix sankaku rating tag count not detected (issue #1312)
• Fix multiple tag conditional filenames (issue #1310)
• Prevent users to choose invalid site apis (issue #1306)