Allow sourcing in any file in .dokkurc

This allows us to configure dokku env-vars individually. Such a feature is useful if a developer would like to enable or disable DOKKU_TRACE without parsing a dokkurc file.
Add a `dokku trace` command to allow easy toggling of the DOKKU_TRACE option
Add a note about enabling `dokku trace` via the cli
Link to gist.github.com
Merge pull request #814 from progrium/dokku-env-vars

Source files in $DOKKU_ROOT/.dokkurc directory and add `dokku trace` command