Support square brackets

• Braces have problem with fish shell (#1). Now fillin supports square brackets like fillin echo [[message]] on fish shell.

Support quoted command

• Support filling and executing a quoted commands (to make it useful for subshells)
• Update some wording of README.md
• Add more tests for scoped variables input prompt

Store filled command history

• Save the executed commands in .fillin.histfile with timestamp.
• Small refactoring.

Improve README, etc.

• stop store the input when all the values are empty
• tweak some samples in README and command help

Stabilize scope grouping prompt

• Improve the behavior of scope grouping prompt
• Improve README.md

Initial Release

• This is the initial release of fillin 🎉