Fix TableFormat for indented tables, version 2

Improved version of 6fe1a10 that
doesn't get fooled by colons.

Author: tomtomjhj

ftplugin/markdown.vim

@@ -550,7 +550,7 @@ function! s:TableFormat()
     " Move colons for alignment to left or right side of the cell.
     execute 's/:\( \+\)|/\1:|/e' . l:flags
     execute 's/|\( \+\):/|:\1/e' . l:flags
-    execute 's/ /-/' . l:flags
+    execute 's/|:\?\zs[ -]\+\ze:\?|/\=repeat("-", len(submatch(0)))/' . l:flags
     call setpos('.', l:pos)
 endfunction