Show [] around string slices when showing the text

This means things like autogenerated docs will show

`--filename=[]`   instead of just `--filename=`

string_slice.go

diff --git a/string_slice.go b/string_slice.go
index bbe6e00..b7096e8 100644
--- a/string_slice.go
+++ b/string_slice.go
@@ -21,9 +21,10 @@
 	return "stringSlice"
 }
 
-func (s *stringSliceValue) String() string { return strings.Join(*s, ",") }
+func (s *stringSliceValue) String() string { return "[" + strings.Join(*s, ",") + "]" }
 
 func stringSliceConv(sval string) (interface{}, error) {
+	sval = strings.Trim(sval, "[]")
 	v := strings.Split(sval, ",")
 	return v, nil
 }