Get profile and config file inheritance working

[ellisonbg]

We need a better way of inheriting profiles and config files. We currently have load_config, but it doesn't handle our new notion of profiles. We probably want to expand this function to an API such as:

load_config('profile_name', 'config_file_name_in_that_profile.py')

[minrk]

added profile keyword arg fror load_subconfig. Now the easy way to inherit default config is:

load_subconfig('ipython_config.py', profile='default')