Add config option: `disable_premailer`

### Requirement

It makes sense for the gem assumes that all emails should be inline CSS. However in our case, we have an existing Rails app that didn't use the gem, but we do want to convert some of our emails to using it. It will be very difficult for us to add `skip_premailer` to each single mailer. So I created this PR to solve this issue

### config option

We add one more config option `disable_premailer` to disable premailer globally. But you can add `enable_premailer` to the header in a mailer to enable the premailer (much like `skip_premailer`, in the opposite direction).

This option is essential for our situation, thanks for the consideration.