Support repeates fields in input

[joshcarp]

This seems to error at the moment

message Foo {
    repeated string field = 1;
}

grpctl .... --field="blah"

Currently this gives a nasty json marshalling error
To fix this either:

• --field="blah" would simply append "blah" to a string array, the same as specifying -H in the root level commands
• an error is returned telling user to supply a json array: ["blah"]

Personally I would prefer the first one, but it would mean that the json creation would need to have context of what type every field is, which is not currently the case