Implementing basic feature configuration section.

Currently supports statically enabling and disabling features. May
eventually support specifying feature methods and other common ways to
dynamically enable and disable features.
Fixing AppSettingsDisabledFeatureIsDisabled fact.

This should fix #1.
Fixing ConfigSectionDisabledElementIsEnabled fact.

This fixes #2.
Adding factory method for features config section.

The new method allows features to be created from the new `<features>`
config section.