Fix NullReferenceException error on opening/closing Untitled

- opening: same as https://github.com/hibara/MarkDownSharpEditor/pull/16
Change the way of syntax highlighter result passing from by field to eventArgs.Result
Merge branch 'hotfix/when-webbrowser-doc-is-null' into develop
Merge branch 'hotfix/backgroundworker2-data-passing' into develop
Add Regex property to MarkdownSyntaxKeyword, and move keyword generating from form to the keyword class