Parsers can now return any number of fields, and can access the whole item

[redhog]

Closes #72

This should be backwards compatible with existing yaml. It also supports the following extended yaml format:

datasets:
  name:
    type: file
    source: local
    path: "file.json"
    parsing:
      - function: function_name
        param_name1: value1
        param_name2: value2
        ...
        param_nameN: valueN

input_key and output_key are supported for all previously existing parsers (and are just normal parameters to them), but might not be supported by any particular new parser. In fact, the llama-index based ones do not support an output_key, but instead places the output in the two fields text and metadata.

For the existing functions that do take input_key and output_key, they are both optional and both have default value "text".