shtab v1.4.2 beta

• 📚 add documentation website https://docs.iterative.ai/shtab (#51)
• 📚 fix subparsers example (#54)
• 🛠️ use string.Template (#53)
• 🛠️ fix py2.7 tests (#53)
• 🛠️ misc test framework updates (#50, #53)