Added in bbcode.cs at line 69(just after "r_bullet":
static private Regex r_tab = new Regex(@"\[tab]", m_options);
Added in bbcode.cs at line 172(just before "// bullets" ) :
bbcode = r_tab.Replace(bbcode, " ");
You can change the width of the tab if you want by adding or removing one or more "nbsp" from the code.