New formatter support

Switch to new html-pretty formatter.

• Easier embedding (no need to adjust ElementTree structure, all needed features are supported by formatter directly)
• Support for "Pseudo steps" (showing before and after scenario as steps)
• Show unexecuted steps after failed step in failed reports
• New lightweight design
• Added auto dark-mode switch
• Added switch to high-contrast
• Easy configuration via behave.ini
• Formatter does not create XML/HTML objects until end of the tests (until formatter.close() call)- lower load between the steps.