Support --root-type when compiling from proto

`--root-type` should generate `root_type Something;` line when used in `--proto` mode

## Expected
```bash
flatc --proto --root-type Something some.proto
```

should build some.fbs that contains line `root_type Something;`

## Actual
--root-type has not effect at all

## Workaround
```bash
flatc --proto some.proto
echo "root_type Something;" >> some.fbs
```