Textual-cogs

Added a demo application called `demo_cogs.py` which shows each of the included dialogs and how to use them. Also added a two new dialogs: `TextEntryDialog` and `SingleColorPickerDialog`.

What's Changed
* Update README.md with screenshot by driscollis in https://github.com/driscollis/textual-cogs/pull/3
* Add the SingleChoiceDialog by driscollis in https://github.com/driscollis/textual-cogs/pull/4
* Refactor dialogs by driscollis in https://github.com/driscollis/textual-cogs/pull/5
* Add demo app by driscollis in https://github.com/driscollis/textual-cogs/pull/6
* Update README.md by driscollis in https://github.com/driscollis/textual-cogs/pull/7


**Full Changelog**: https://github.com/driscollis/textual-cogs/compare/v0.0.2...v0.0.3

This is the initial release of `textual-cogs`, an add-on package for [Textual](https://textual.textualize.io/) that is currently focused on providing simple dialogs to make your TUI development easier. In this release, you get the following dialogs:

- Generic `MessageDialog` - shows messages to the user
- `SaveFileDialog` - gives the user a way to select a location to save a file
- `TextEntryDialog` - Ask the user a question and get their answer using an `Input` widget

Enjoy!