Please install `install_requires` at setup time

I find that I have to duplicate my `install_requires` in my `setup_requires` when I use `argparse_manpage`. This makes sense: my code needs its `setup_requires` to be run by `argparse-manpage`.

It would make sense for the `argparse-manpage` plugin for `setuptools` to install the `install_requires` in the setup phase too.