Feature: profile ini flag

This is a first run at fixing https://github.com/Pylons/pyramid_debugtoolbar/issues/172. At the moment it requires knowing the internal `dom_id` of the panel to activate it from the `.ini` file.

There is no good way to currently get the registry for the debugtoolbar into the panels themselves from what I can figure out.

The `DebugToolbar` class gains a new `registry` argument that is set to `None` by default. There may be a better way to do this.

Please take this as a starting point for improving upon.