V3.1.1 bug fix release and performance

What's Changed

• V3.0 by @tbela99 in #160
• fix installation issues by @tbela99 in #162
• better automatic critical css generation
• fix css parsing and rendering issues
• fix gif to webp conversion issues

Full Changelog: V3.1...V3.1.1

#164 fix fatal error and notices when processing invalid images 58c2562
#164 fix multiple bugs 60fafa3
change version number d58b45b
fix merge conflict #164 873f692
#164 incorrectly parsing @ charset 82ed83a
#164 compute file hash only based on its content to avoid performance… af51a11
#164 revert the change. the hash computation method is configurable t… 79e7040
#164 fix CSS bugs 6823eac
load critical css generator only when required #164 ac58ae3