Add Plugin IF for html and svg rendering.

A MdXaml plugin add syntaxes and additional supported image formats. For now, there are two official plugins; MdXaml.Html to add html tag support and MdXaml.Svg to add SVG support.

If you want use it. Add nupkg MdXaml.Plugins and MdXaml.Html, MdXaml.Svg, and edit App.xaml like the bellow sample.

<Application x:Class="***"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mdplug="clr-namespace:MdXaml.Plugins;assembly=MdXaml.Plugins"
             xmlns:mdhtml="clr-namespace:MdXaml.Html;assembly=MdXaml.Html"
             xmlns:mdsvg="clr-namespace:MdXaml.Svg;assembly=MdXaml.Svg"
             StartupUri="***">
    <Application.Resources>
        <mdplug:MdXamlPlugins x:Key="MdXamlPlugins">
            <mdhtml:HtmlPluginSetup/>
            <mdsvg:SvgPluginSetup/>
        </mdplug:MdXamlPlugins>
    </Application.Resources>
</Application>