Do any rabbinical sources hold that the commandment in Leviticus 19:18 to love one's neighbour as oneself (ואהבת לרעך כמוך) applies to non-Jews as well as Jews?
Although the term רעך (your fellow/neighbor, friend) is generally understood as applying strictly to one's fellow Jew, R' David Sears brings a number of sources related to loving every one, including one which see a broader application of this verse in his book Compassion for Humanity in the Jewish Tradition:
Love for one's neighbor means that we should love all people no matter to which nation they belong or what language they speak. For all men are created in the Divine image and all engage in improving civilization...Our love of humanity should take no exception to any nation or individual....Therefore, not only does [love of one's fellow] apply to the Jewish people but to all mankind. We should love all nations and include all peoples in this universal principle, "the stranger and the native son" alike, all who inhabit the earth. Let every man strive for the benefit of his fellow, in a spirit of mutuality, whether in physical concerns or in financial matters, for the collective good and for the improvement of society. Loving one's neighbor means that we should befriend all human beings. (Rabbi Pinchas Eliyahu of Vilna, Sefer HaBris, sect. II, discourse 13)."(page 6 and 7)
He subsequently relates a story about Rabbi Pinchas of Koretz who was very pleased when presented with a sefer teaching this idea, presumably the aforementioned Sefer HaBris.