Add new option "preserveClassAttribute" to preserve class attribute on elements

Gmail for example will condense an email if it is over a certain length. Removing class names after juice is run would help decrease the length drastically.

No file or test is named. Start by locating how Juice options are parsed and how class attributes are handled during CSS inlining; then inspect the existing tests before deciding where coverage belongs. The work is done when the option preserves class attributes while the default behavior remains unchanged.

Written by the indexing model from the issue text.