Clean up, improvements to evaluation and a new help system

Liquid 0.8.0 contains many minor bugfixes, improvements to code evaluation, a new help system and some clean up. The clean up part involves some breaking of backwards compatibility. Some features like vim- and emacs keybindings have been removed. They can still be accessed through external extensions. The clean up is done to keep the code tight and as part of working towards a 1.0 release with a very stable and clean core prepared for extensions. The primary outstandings for a 1.0 release is documentation, guides, tutorials and changes needed to support extensions in a nice way.