Allow for custom pip flags to be passed to the pip command

This commit allows custom pip flags to be passed on to the pip executable. My usecase was disabling the inclusion of pyc files in the generated package by passing on the --no-compile pip arg.
Few other flags, like verbose, are handled above in the `create_virtualenv` method by appending (or in this case, extending) the pip prefix. Could you move this there too?

You should probably use extend too, because appending might cause mis-quoting in the subprocess, because subprocess will quote, when given an array, every element independently e.g. multiple options might end up inside same quotes when executing:

 ```python
    extra_pip_args = ['--no-compile', '--extra-index-url=derp']
    self.pip_prefix.append(" ".join(extra_pip_args))
    subprocess.check_call(self.pip_prefix)
    # This probably results to a call of style: pip --someswitch "--no-compile --extra-index-url=derp"
```

Unfortunately this behaviour is bit hard to test without actually running some commands