Load theme data via .json file

[Ephenodrom]

Add functionality to manage theme data via .json file.

Load json file at startup

It should be loaded if no MaterialTheme or CupertinoTheme is given to the EzRunner.

Convert json data to theme data

https://api.flutter.dev/flutter/material/ThemeData-class.html

• Convert Colors
• Convert textTheme
• Convert iconTheme

Add theme to MaterialApp / CupertinoApp

Add it after successful parsing.

[robrbecker]

FYI There appears to be a package that could help with this enhancement https://pub.dev/packages/json_theme