added `caldera_forms_mailer_nl2br` filter to prevent undesired `<br>` tags

I had some issues with the mailer feature using the visual editor to build the body of emails with tables, where the `nl2br()` function was adding a `<br>` tag for each table line. This pull request keeps the original behavior, but gives the user a way to remove this behavior if is outputting undesired tags.