Fix reserved name used for module.

I'm a lazy man and decided that I don't need to remember how Python's
modules and packages work because hey, this is a small tool and all.

I am also not a smart man and got bit by naming a module vars, which
is also a Python built-in function.

Signed-off-by: Weland Treebark <weland@blinkenshell.org>
Removed doc reference to date variable.

This feature won't actually get implemented, since it's quite
superfluous.

Signed-off-by: Weland Treebark <weland@blinkenshell.org>
Crank: read .blag variables.

Variables defined in .blag are now read before cranking the posts.
This is required for processing the post, as <bb:var> tags can
reference these.

Signed-off-by: Weland Treebark <weland@blinkenshell.org>