BUG: Support local pip packages and editable installs

[HaoZeke]

Consider the following:

    "matrix": {
        "req": {
            "numpy": [""],
            "pip+asv_runner": ["asvWork/asv_runner"]
        },
    },

Which currently doesn't work. Indeed, any kind of options are also not passed through to pip, making -e also not possible to be passed. This is contrary to the documentation, which leads one to believe that the arguments are passed to pip transparently.