Add support for --download

[tlynn]

peep adds its own --download argument when calling pip, and gets all confused if the user provides one too. (The motivating use-case is automatically preparing a directory of known-good files ready for e.g. building a native DEB/RPM package).

[willkg]

@tlynn For clarity sake, is this the peep invocation you're having problems with?:

$ peep.py install --download -r requirements.txt

If not, what does the invocation look like?

[tlynn]

peep install --download dir -r requirements.txt

See "pip install --help".