Fix docs and code regarding default formatter.

"nice" was the default for a looong time, but it's default value of the "-f"
flag. Function "get_formatters" returns "vim" in case if it fails to find one.
It should return "nice" as well. This patch fixes it.

docs/autocomplete_formats.md

@@ -1,7 +1,7 @@
 
 # Description of Completion Assistance Formats #
 
-Use `-f` parameter for `autocomplete` command to set format. VIM format is default and fallback.
+Use `-f` parameter for `autocomplete` command to set format. "nice" format is the default and fallback.
 
 Following formats supported:
 * json

formatters.go

@@ -163,5 +163,5 @@ func get_formatter(name string) formatter {
 	case "godit":
 		return new(godit_formatter)
 	}
-	return new(vim_formatter)
+	return new(nice_formatter)
 }