feat(display): add methods to display

- Add `indent`, `faint`, and `underline` methods to `display`
- Add tests for the above

internal/display/indent_test.go

@@ -0,0 +1,12 @@
+package display
+
+import "testing"
+
+// TestIndent asserts that Indent prepends a tab to each line
+func TestIndent(t *testing.T) {
+	got := Indent("foo\nbar\nbaz")
+	want := "\tfoo\n\tbar\n\tbaz"
+	if got != want {
+		t.Errorf("failed to indent: want: %s, got: %s", want, got)
+	}
+}