Highlighting engine should support a way to end inner spans from when ending an outer span

[dgrunwald]

SD-1919, originally created on 3/17/2013 17:43:45 by Siegfried Pammer

In MarkDown, for example, the current implementation of Spans is not
sufficient, if we mix MarkDown with C# as Code-RuleSet.

this.SomeCall() // this comment

In this example the matching ` should end the nested //-span.
To preserve features such as nesting multiple multi-line comments (like
in Boo) we should introduce a new attribute which marks a span with a
priority index. All spans with lower priority should be closed, if the
outer span is closed.