Add a feature that parses parameters of a url.

Add a feature that parses parameters of a url. After parsing, each parameter is given to JSON value.
A url to be parsed is declared through :parameter_convert directive.
If you want to change a key name, you can define the mapping from old names to new names through :parameter_mapping directive.


ex)
fluentd.conf:
<source>
  type tail
  ...
  parameter_convert referer
  parameter_mapping {"foo":"bar"}
</source>

When referer is like: "http://sample.com?foo=1",
then fluentd parses it and generates value like:
  {"bar": 1}