Added highlight_style For easy access to themes

[Abbas1Hussein]

Usage Example

return HighlightView(
  // The original code to be highlighted
  code,

  // Specify language
  // It is recommended to give it a value for performance
  language: 'dart',

  // Specify highlight theme
  theme: HighlightStyle.github.theme,

  // Specify padding
  padding: EdgeInsets.all(12),

  // Specify text style
  textStyle: TextStyle(
    fontFamily: 'My awesome monospace font',
    fontSize: 16,
  ),
);