Syntax: Fix setext headings in symbol list
Syntax: Fix setext headings in symbol list

SETEXT headings need special treatment to remove underlines from symbols
in symbol list.

This commit merges in changes from ST's default Markdown package and
fixes SETEXT headings.

Before this commit they looked like:

   heading======

With this commit:

   heading

They are converted to ATX style in global index.

SublimeText-Markdown/MarkdownEditing