Add as_tuples argument to spaCyLayout.pipe

[ines]

Add option for as_tuples in spaCyLayout.pipe, analogous to spaCy's Language.pipe.

layout = spaCyLayout(nlp)
sources = [("./foo.pdf", {"baz": 1}), ("./bar.pdf", {"baz": 2})]
for doc, context in layout.pipe(sources, as_tuples=True):
    ...