Add methods in JsonParser, to "push back" buffered but unused content

Jackson JsonParser buffers its input extensively, since this is beneficial for performance. But once read into buffers, there is no way to "push back" content that is not parsed. Being able to do that would be very useful for cases where JSON content is interleaved with other content.

Since there are two kinds of input sources (byte-based like InputStream, character-based like Reader), there need to be two methods, for example:

JsonParser.releaseBufferedInput(OutputStream);
JsonParser.releaseBufferedInput(Writer);

when called for appropriate type, all currently buffered but unused content should be written using given argument.