Add autoenv add=local subcommand

The `add=local` subcommand creates `.local/bin` if it doesn't exit and
then updates the autoenv to add it to `PATH`.

autoenv/_autoenv

@@ -17,6 +17,7 @@ _autoenv() {
         edit:'edit .enter and .exit scripts in current directory'
         deinit:'remove .enter and .exit scripts in current directory'
         reload:'reload the current environment'
+        add=local:'add .local/bin to PATH'
         add=py:'add Python virtualenv to the autoenv'
       )
       _describe -t commands command commands && ret=0 ;;