Allow for renaming/disabling of origin param

[tmilewski]

This PR allows for renaming or disabling of the origin param per the issue (#910) that @97jaz submitted. [Backwards Compatible]

Examples

Default Flow:

# https://www.domain.com/auth/twitter/?origin=[URL]
# No change

Renaming Origin Param:

# https://www.domain.com/auth/twitter/?return_to=[URL]
provider :twitter, ENV['KEY'], ENV['SECRET'], origin_param: 'return_to'

Disabling Origin Param:

# https://www.domain.com/auth/twitter
# Redirects handled externally, `omniauth.origin` is not set
provider :twitter, ENV['KEY'], ENV['SECRET'], origin_param: false