Create a BlockBody class to decouple block body parsing from tags.
Avoid storing options instance variable in BlockBody.

There is no need to pass parse options to the BlockBody initializer, since
it does all the parsing in the parse method, unlike tags which parse the
tag markup in the initializer.
Remove unused local variable that was accidentally added.
Merge pull request #458 from Shopify/block-body

Create a BlockBody class to decouple block body parsing from tags.