allow_network=False prevents loading of local stylesheets

[evanwall-carta]

If Premailer is initialized with allow_network=False then it will not be able to load local stylesheets.
The following will be ignored by Premailer:

<link rel="stylesheet" href="email/base/base.css">

The root cause of this bug is probably here:

premailer/premailer/premailer.py

Line 374 in f4ded0b

cssselector.append("link[rel~=stylesheet]")