sanitize: naive CSS sanitizer implementation

- CSS sanitizer allows a limited set of properties in a style attribute.
- Added a CSS inlined version of the tutsplus responsive test mail.
- Linter fixes in inbucket.go

sanitize/css_test.go

@@ -0,0 +1,34 @@
+package sanitize
+
+import (
+	"testing"
+)
+
+func TestSanitizeStyle(t *testing.T) {
+	testCases := []struct {
+		input, want string
+	}{
+		{"", ""},
+		{
+			"color: red;",
+			"color: red;",
+		},
+		{
+			"background-color: black; color: white",
+			"background-color: black;color: white",
+		},
+		{
+			"background-color: black; invalid: true; color: white",
+			"background-color: black;color: white",
+		},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.input, func(t *testing.T) {
+			got := sanitizeStyle(tc.input)
+			if got != tc.want {
+				t.Errorf("got: %q, want: %q, input: %q", got, tc.want, tc.input)
+			}
+		})
+	}
+
+}