Support config.templates.linenums

This option allows to show line numbers on source code.
The option is the same as used in https://github.com/terryweiss/docstrap
Merge pull request #13 from vmeurisse/linenums

Support config.templates.linenums
A better template for exceptions

Instead of a non-styled dd/dt template, called for each exception, call
one template with a table for all exception, using same styles as
params: type and description