Add a logfmt formatter

[ltrzesniewski]

Add a LogfmtFormatter which produces logfmt output, for easy parsing by log aggregators.

This would output some standard fields. Make their keys configurable through init-only properties with suitable defaults. Suppress them if the configured key is null or empty.

Append metadata as their own key/value pairs.

Open questions:

• What keys should we use for the standard fields by default? Proposal: date, level, thread (or none by default?), logger ,msg
• In what order should we output the standard fields? Same order as above?
• How do we escape double quotes in values? With \"?
• What about multiline values?
• What if a metadata key contains a space or other character that needs escaping? Or even a newline? Maybe just drop it?
• How to output exceptions? Just the message or the whole (multiline) stack trace?
• What date format to use? yyyy-MM-dd HH:mm:ss.fffffff?

/cc @oktal