Ruby layer, tweaks and fixes

• New contrib layer ruby (thanks to bru)
• Enable semantic in python layer (thanks to danielwuz)
• Add evil key binding z f which recenters the window to match as much as possible the function at point, very handy! (thanks to danielwuz)
• Correctly execute clear and man commands in shell (thanks to avendael)
• New conventions in CONVENTIONS.md: REPLs, Test execution, code navigation and documentation.
• Various tweaks and fixes to layer READMEs