Pass per-target options as invoke args

[edvardm]

If we have a rule like

foo: opts ?= --fast -W
foo:
    build $(opts)

we could try parsing those opts and pass to invoke with some assumptions like this:

@task
def foo(c, fast: bool = True, W: bool = True):
    c.run("build")

Not sure if this is the way to go though, as the two formats are not isomorphic; using example above, Make would treat opts just as an unstructured string allowing use of positional args, which invoke doesn't have