add remote status, add and remove commands

Expose status, add and remove through cmdliner, each defaulting to the
origin remote and accepting --remote. Expected failures print one line on
standard error and exit 1 instead of raising.

bin/main.ml

@@ -1,2 +1 @@
-let () =
-  exit (Cmdliner.Cmd.eval Git_fleet.Cli.cmd)
+let () = exit (Cmdliner.Cmd.eval' Git_fleet.Cli.cmd)