@blocktag no longer hard-codes end as the prefix for the end tag. A

caller can pass end_prefix="end_" or similar to @blocktag, which is

useful when naming a function with underscores.
Decorators are expected to act differently depending on whether they're

called in @decorator form or @decorator() form. Rather than breaking

every single usage of @blocktag by requiring parens (since we now have

an optional argument), we have to do some checking of arguments at the

end to determine what to return. This is pretty harmless in this case,

but worth pointing out.

Tested with a new template tag I'm writing, which uses end_prefix.