go-srslog-lib

Add ErrNilDialFunc and return it from DialWithCustomDialer. We add it here instead of the customDialer method since it only needs to be checked once, not every time a dial attempt is made, and it is similiar to other checks made by Dial* funcs

Add DialFunc type in place of repeating dial function signature

Rename cdialer in Writer struct to customDial, was going to use customDialer but that conflicts with the method of the same name