Add --show-ast command line option

I guess I didn't document it properly, but `ast_dump` prints only one single node, along with it's children, from the ast at the `pos` position. Usually this node is a `SCRIPT` node, that is the root of the tree. Hence if you invoke `aparse` multiple times on the same AST buffer, `ast_dump` will only print the first one.

I'd go for simply reinitializing the ast buffer before each parse and dumping right afterwards.