keyval_decoder: convert 'MSG a=b c=d' to title and json

- support for quote-escaped values
- set payload to stringified json
- pass message fields via heka TOML
- ParseTitleAndKeyvals handles '\n'
Typically you'd have this declared at the top level, so it didn't get compiled every time you declare this function

```go
var keyValRegex = regexp.MustCompile...

func findKeyValPair....
``