Allow more customization on if-conditional whitespace.

Replace IfStatementConditional with separate IfStatementConditionalOpening
and IfStatementConditionalClosing to allow whitespace insertion inside the
conditional.

This would break existing configurations that don't know about the new settings, but I didn't see any way to make this work with the existing properties. I've adjusted the config in the existing custom test, but didn't touch the files itself, they didn't change.

I've got something similar in the works for for-in statements. Required for #19.