[Feature Request] Add logfmt to decoder

In flb_parser_decoder.c
only support decoder: json, escaped, escaped_utf8

I need add logfmt (flb_parser_logfmt.c) to decode simple json:

`
{ "log": "a=b c=d" }
`
need get key a=>b, c=>d.
If I use parser logfmt for json string, i get in stdout:

My stdout:
`[OUTPUT]
Name stdout
Match *
Format json_lines
json_date_key time
json_date_format iso8601
`
my output:
`
{"time":"2019-04-19T16:15:04.000000Z", "{":true, "log":true, ":":true, "a":"b", "c":"d", "}":true}
`

Please help with add logfmt in decoder...