Feature request: ability to set default formatter

Hi, thanks for maintaining this library

I'd like to use this library to return errors to a caller that would prefer no newlines if possible (in the event of one error). That is doable and looks like this:

```
func Foo() error {
var retErr *multierror.Error
for _, x := range getSlice() {
if err := bar(x); err != nil {
retErr = multierror.Append(retErr, err)
}
}
return decorateFormat(retErr.ErrorOrNil())
}

func decorateFormat(err *multierror.Error) *multierror.Error {
if err != nil {
err.ErrorFormat = multierror.ErrorFormatFunc(func(errs []error) string {
if len(errs) == 1 {
return errs[0].Error()
}
return multierror.ListFormatFunc(errs)
})
}
return err
}
```

Which is fine, but requires me to remember to set the listFormatFunc, I'm doing that in a separate "decorator" func because I will need to do this many times.

I would like to make it possible to specify the "default" error func so I don't have to remember to set it. This way, whenever a new multierror.Error was created, it would automatically have the format function I want. Would you accept a change like that? If so, perhaps a default global (defaults to the current `ListFormatFunc`) is the way to go, but would defer to you. Thanks in advance!