Add scope/field selector settings for LinkedIn provider.

This change adds two configuration variables to third_party_auth's LinkedIn provider:
SOCIAL_AUTH_LINKEDIN_OAUTH2_SCOPE and SOCIAL_AUTH_LINKEDIN_OAUTH2_FIELD_SELECTORS.

Being able to configure these two additional settings is useful
if you want the LinkedIn provider to pre-populate the email field
when a new user registers via the linkedin provider.

The Google provider prepoulates the email field by default, but if you
want LinkedIn to do the same, these two settings should be set to:

SOCIAL_AUTH_LINKEDIN_OAUTH2_SCOPE = ['r_basicprofile', 'r_emailaddress']
SOCIAL_AUTH_LINKEDIN_OAUTH2_FIELD_SELECTORS = ['email-address']

For more info see:
http://psa.matiasaguirre.net/docs/backends/linkedin.html