Adding field to_string/from_string methods that encode to JSON/YAML strings.

@cpennington I understand what you mean, although some kind of indentation is still necessary as readability is why we actually do this. (Otherwise, we could just put everything into an extremely long attribute.)

Although It's possible to construct a to_string method to a custom field, what we have so far are JSON, where whitespace is not meaningful. (Newlines in a string are escaped along with all control characters.)

I looked for removing one of the functions, but I did not find an indenting tool in the standard library, or a function in lxml to get the depth.