Support separate formatters, overriding skip for pretty printing mode

[mzabaluev]

It would be nice to be able to declare different formatting for the pretty printing representation.
For example, fields that are skipped in normal mode could be printed in the pretty mode:

#[derive(CustomDebug)]
TestingError {
     message: String,
     #[debug(skip)]
     #[pretty]
     logs: Vec<String>,
}

[panicbit]

I like this idea, but I'll have to think about how I want this to work.

[mzabaluev]

I think it would be fine if the pretty field attribute accepted the same parameters as debug, except that the default pretty formatting would be taken from the debug attribute if present on the field, and pretty without arguments would invoke the default Debug formatting in the alternate mode even if debug(skip) is present.