Make --expect additive

Similarly to --bind or --color.

--expect used to replace the previously specified keys, and
fzf#wrap({'options': '--expect=f1'}) wouldn't work as expected. It
forced us to come up with some ugly hacks like the following:

https://github.com/junegunn/fzf.vim/blob/13b27c45c8bdf6c3a41376bb83e4895edadf8c7e/autoload/fzf/vim.vim#L1086

src/options_test.go

@@ -414,3 +414,10 @@ func TestPreviewOpts(t *testing.T) {
 		t.Error(opts.Preview)
 	}
 }
+
+func TestAdditiveExpect(t *testing.T) {
+	opts := optsFor("--expect=a", "--expect", "b", "--expect=c")
+	if len(opts.Expect) != 3 {
+		t.Error(opts.Expect)
+	}
+}