0.6.0

• Added MongolRichText with support for TextSpan and its substring text styles.
• Added MongolText.rich constructor to support using a default font theme.
• Implemented textScaleFactor. It existed before but didn't do anything.
• Removed the default Menksoft font that was included with the library. This creates a bit more setup but makes the library smaller and allows developers to use any Mongolian font. Also removed MongolFont class.
• Added documentation modified from the standard Flutter docs.
• Changed the license to be in alignment with the Flutter BSD-3 license.
• Updated the demos to include a keyboard and vertical TextField. These are not in the library yet, but should be fairly easy to reproduce by studying the demos.
• Added more tests.