Make fzf pass go vet

Add String() methods to types, so they can be printed with %s. Change
some %s format specifiers to %v, when the default string representation
is good enough. In Go 1.10, `go test` triggers a parallel `go vet`. So
this also makes fzf pass `go test`.

Close #1236
Close #1219

src/ansi_test.go

@@ -26,7 +26,7 @@ func TestExtractColor(t *testing.T) {
 		output, ansiOffsets, newState := extractColor(src, state, nil)
 		state = newState
 		if output != "hello world" {
-			t.Errorf("Invalid output: %s %s", output, []rune(output))
+			t.Errorf("Invalid output: %s %v", output, []rune(output))
 		}
 		fmt.Println(src, ansiOffsets, clean)
 		assertion(ansiOffsets, state)