Support sub-parsing by "start in Pod" mode

[leonerd]

I'm attempting to parse the contents of a =for ... block for my own formatter, partly by using an inner Pod::Simple instance. By default a newly-created instance does not start in pod mode, it presumes code mode. So when I want to perform an inner sub-parse of the captured contents of the block, I have to prepend "=pod\n\n" on that text to make it work. It's a bit of a hacky workaround; it'd be nice if there was some constructor argument or method I could poke to knock it into Pod mode immediately.

[khwilliamson]

patches welcome