User-defined cell renderers not parented in the view tree

Cells rendered via a user-defined template rendered provided with the "tmpl" option are not parented in the view tree. This makes it impossible to look up information on parent template instance data using, say, aldeed:template-extension. Note that the "tmplContext" function is not a substitute since you can't determine the particular part of the view tree the DOM is attached to.

Simple patch attached.
[Include_user_defined_cell_templates_in_the_view_tree.patch.txt](https://github.com/aldeed/meteor-tabular/files/791934/Include_user_defined_cell_templates_in_the_view_tree.patch.txt)

Start by reviewing the attached Include_user_defined_cell_templates_in_the_view_tree.patch.txt and tracing the user-defined template path using the "tmpl" option. Done means cells rendered from user-defined templates are parented in the view tree so parent template instance data can be looked up through the DOM attachment.

Written by the indexing model from the issue text.