Add the linphone_core_new_with_config() function to instantiate a LinphoneCore given an already existing LpConfig.

This enables the creation of the LpConfig before creating the LinphoneCore.
It is useful on some systems to read some configuration parameters and perform some customization before creating the LinphoneCore.

The LpConfig can now also be created given a factory config filename using the new lp_config_new_with_factory() function.