Add Support for Golang's logfmt

[blaggacao]

Please consider adding a parser, similar to json, for https://godoc.org/github.com/kr/logfmt
which is the default way of logging in everything golang, if no TTY is attached.

About 40% of my infrastructure containers speak this format, this might be quite average in some containerized environments, where everything's golang now a days...

[blaggacao]

@edsiper I'd put a bounty out of current budget of USD 150,- on it. Let me know if and where to reward...

[edsiper]

@blaggacao please coordinate with @manuelluis, he just implemented this feature.

[blaggacao]

Great news. @manuelluis, could you contact me on [email protected] for tx details. You should expose your signature attached to GitHub in any way to authenticate it's you.

[bluebike]

Just comment to current structure of parsers.

I'm implementing parser for MariaDB audit logging currently it's just a filter,
but I could make that parser also, becuase it can parse (some) csv-files.

Currently parsers are "hardcoded" in flb_parsers.c/h.
Maybe parsers should be refactored to plugins (or similar structure).
There should be some kind of init/config, execute, close methods for those.