Added #subpage functionality and various minor fixes
* improved error reporting on syntax failure
* fixed small js scope issue

Given the subpages A/B/C

<sidebarmenu>
 #subpage A
</sidebarmenu>

would render a sidebarmenu with the same hierarchical structure
as the subpages equivalent to the input:

<sidebarmenu>
A
*B
**C
<sidebarmenu>

Subpages can also be included anywhere in the tree structure
E.g

<sidebarmenu>
My subpages a couple of levels down
* A tree of subpages is closing...
** A tree of subpages is IMINENT!
*** #subpage A
</sidebarmenu>

Additionally subpages can be prefixed with -/+ indicating wether
the entire tree and the children should be collapsed or expanded

Change-Id: Ifaf87db3bd0a86aa871022d7c906050e95269720