Support for textprotos

As far as I understand this library, it allows only to read protos from and into bytes. I would be useful to also add text proto support which is sometimes used for small config files.

It might look like this:
```
person {
  name: "John Doe"
  email: "jdoe@example.com"
}
```
or this:
```
person <
  name: "John Doe"
  email: "jdoe@example.com"
>
```