let me set a default charset

I'd prefer iso-8859-1, let me set it without doing monkeypatches

@jeremy @bf4

atm I'm using this:
```Ruby
module DefaultCharset
  def init_with_string(*args, &block)
    @charset = 'iso-8859-1'
    super
  end
end

Mail::Message.prepend(DefaultCharset)
```
So, we're starting to get some duplication here

https://github.com/mikel/mail/commit/8ad1b8d56b04a67ee4853093a55bea73a2ee7c27#diff-93830fa29d616f7c87903d08b5b1b29aR33

```ruby
if defined?(Encoding) && Encoding.respond_to?(:default_external=)
  begin
    old, $VERBOSE = $VERBOSE, nil
    Encoding.default_external = 'utf-8'
  ensure
    $VERBOSE = old
  end
end
```

https://github.com/mikel/mail/blob/8ad1b8d56b04a67ee4853093a55bea73a2ee7c27/lib/mail/parsers/ragel/ruby.rb#L5

```ruby
        def self.silence_warnings
          original_verbose = $VERBOSE
          $VERBOSE = nil
          yield
          $VERBOSE = original_verbose
        end
```