Add options to disable default syntaxes

[chenzhiguang]

Fixes: #393

Example

const text = '<center><a>**some text**</a></center>';

final result = markdownToHtml(
  text,
  withDefaultInlineSyntaxes: false,
  withDefaultBlockSyntaxes: false,
  encodeHtml: false,
  inlineSyntaxes: [
    TagSyntax(r'\*+', requiresDelimiterRun: true),
  ],
);

print(result);

[srawlins]

This looks great! We need some tests. Maybe something simple that:

• no syntaxes; what does that look like?
• no inline syntaxes
• no block syntaxes

[srawlins]

Great tests, thanks!

[srawlins]

Excellent