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.
I think that's a tabbing issue in your editor? Vim does the same thing for
me (shows the whitespace mismatch). However, If I render the docs, they
look OK on my end. Could you confirm this?

Attached a couple of screenshots illustrating this:




On Tue, Sep 2, 2014 at 11:37 AM, Jyrki Pulliainen <notifications@github.com>
wrote:

> In doc/dh_virtualenv.1.rst:
>
> > @@ -44,6 +44,7 @@ OPTIONS
> >  --extra-index-url			Pass extra index URL to pip
> >  --preinstall=PACKAGE			Preinstall a PACKAGE before
> >  					running pip.
> > +--extra-pip-arg				Extra arg for the pip executable.
>
> Here's some tiny whitespace mismatch
>
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> <https://github.com/spotify/dh-virtualenv/pull/58/files#r16975669>.
>