Add a CustomString type to allow unescaped output.

[kormat]

When trying to print multi-line (or otherwise formatted) messages, the fact that all strings are escaped becomes a problem. For example:

func PanicLog() {
    if err := recover(); err != nil {
        log.Crit("Panic", "stack", string(debug.Stack()))
    }
}

The stack trace becomes a single line, with all \n's and \t's escaped. With this new type, the central line becomes:

log.Crit("Panic", "stack", log.CustomString(debug.Stack()))

and now it formats in a readable way. Any CustomString values are surrouneded by triple backticks to it easier to see where they start/end. An example output is viewable here: https://gist.github.com/kormat/10e09e4dcfef0115c65fcdc20c98297e

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This change is 

[ChrisHines]

Triple back ticks are not part of what is currently understood as the logfmt format and are not understood by the existing Go logfmt parsers in github.com/kr/logfmt and github.com/go-logfmt/logfmt. I doubt that the logfmt parsers written in other languages will parse triple back ticks either.

A log15.CustomString that itself contains triple back ticks would result in an ambiguous log entry.

Using the proposed log15.CustomString type would help human readability, but harm machine readability.

[kormat]

Hmm, k. Looking at kr, i don't see any way to support making multi-line output readable for humans. Assuming that is the case, do you think it would be possible to get it merged anyway, with a warning saying that this won't work for machine parsing? We use multi-line logging a Lot in our code.

[kormat]

It wouldn't be hard to make it machine readable (e.g. by prefixing all of the lines with something like >), just not within logfmt.

[kormat]

I've updated the PR and gist with a machine-readable representation.

[kormat]

Rebased and updated. We've been using this for the last 2 years, and have tooling which parses the output.

[davisford]

LGTM - I want an option to print un-escaped strings to the console for many reasons.