Add missing loader option for the parser

[draringi]

(At least) One way of loading data into the parser is missing. ucl_parser_add_chunk is similar to ucl_parser_add_string, except len=0 does not imply a zero terminated string.
In addition, ucl_parser_add_chunk_priority does the same, but with a priority. The use of this is not yet clear in the documentation.

Furthermore, ucl_parser_add_fd adds a file by file descriptor, so it may be possible to open the file in Go, and pass the file descriptor (to investigate)