0.7.0: prebuilt plugins, activation in a new shell and more

• on/activate/use: now works in a new shell by default
• on/activate/use/mk: added option --same-shell
• cache redesign: splited into sub-dirs {arc,src,build}
  • added migration script for the new cache
  • common: added get_(build|arc|src)_path functions
  • fixed commands rm-cache/ls-cache
  • elixir plugin: used get_(build|arc|src)_path instead of nv_get_cache_full_path
  • julia plugin: used get_(build|arc|src)_path instead of nv_get_cache_full_path
• added nv_get_system into common
• mk: added --force option for re-create environment
• mk: added reset for plug_state
• mk: added clean src folder after build (for saving disk space)
• added plug_build function for usage in plugins
• added plug_build_env function for usage in plugins
• renamed plugin go into go-prebuilt
• added plugin node-prebuild
• added plugin rust-prebuilt
• added plugin elixir-prebuilt
• improved bash autocompletion for commands on/activate/use/do