Add Plugin IFs and syntaxes

Plugin IF

A new plugin IF is provided that adds extended syntax and image format resolvers.
This project provides the following plugins.

• Markdown.Avalonia.Svg: Supports SVG image format.
• Markdown.Avalonia.Html: Supports HTML tags (not complete. Only some tags are supported)
• Markdown.Avalonia.SyntaxHigh: Customise syntax highlighting rules.

New syntax

Markdown.Avalonia has color-text syntax; eg. %{color:red}colortext%.
From this version, you can add background indication.

This means that the following notations will be available.

1. %{ color : red ; background : yellow }some word%
2. %{ background : yellow ; color : red }some word%
3. %{color : red}some word%
4. %{background: yellow}some word%