TypeScript rewrite

• Ported to TypeScript, yarn and modern packages (#18)
• Renamed --static_dir to --static-dir and made it default to the template directory (rather than current directory)
• Added --write-template flag to produce sample templates (#19)
• Added general support for keyboard shortcuts via <input data-key="k" ...> (#19)
• Show a warning when the submission contains no new data (#6)