Document how boolean flags work

[janderland]

It took me some time to figure out how the boolean flags work. The flag --truncate is set to true by default, but the CLI help message only includes info about what this flag does and lacks information on how to set it to false.

I went into the humanlog source code and opened up the docs for the latest version of the CLI library it uses (urfave/cli). This led me to try the flag with the no- prefix. This didn't work because humanlog isn't using the latest version of the library.

I also tried --truncate false which didn't work.

I finally found an issue which describes flags being used as --flag=true which led me to try --truncated=false.

Ideally, I wouldn't need to search through the issues of a dependent library to figure out how to use humanlog.

Thank you very much for all the work you've put into this tool.

[aybabtme]

Thanks, I'll look at making this better. It trips me up as well.