Add support to allow Tag Helpers to generate content before and after source element

[DamianEdwards]

Tag Helpers can specify content to appear before and after the contents of the HTML element they're operating on. Sometimes however, a Tag Helper will want to generate content that appears before or after the element itself, e.g. the <link> and <script> Tag Helpers in MVC. Currently, that requires them to manually rebuild the original element by suppressing the default output and constructing the original HTML using the information from TagHelperContext and TagHelperOutput. This should be simpler for Tag Helpers to do.

We should add new properties to TagHelperOutput that enable this, e.g.:

public TagHelperContent PreElement { get; }
public TagHelperContent PostElement { get; }

Example

Source file:

<my-tag-helper>
    Content in source
</my-tag-helper>

Tag Helper file:

public void Process(TagHelperContext context, TagHelperOutput output)
{
    var nl = Environment.NewLine;
    var br = "<br />" + nl;
    output.PreElement.Append("This will appear before source element" + br);
    output.PreContent.Append(nl + "This will appear before source content" + br);
    output.PostContent.Append(br + "This will appear after source content" + nl);
    output.PostElement.Append(br + "This will appear after source element");
}

Output:

This will appear before source element<br />
<my-tag-helper>
This will appear before source content<br />
    Content in source<br />
This will appear after source content
</my-tag-helper><br />
This will appear after source element

[NTaylorMullen]

c9c8e80

aspnet/Mvc@2c4c35e