better handling of configuration actions in the conf object

[Alfredo Deza]

One way to alter configuration values in Pecan, for example for testing, would be something like:

from pecan import conf, configuration

# alter a value
conf.foo = 1

# set it
configuration.set_config(conf, overwrite=True)

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It is a bit confusing to have different modules to work with. This is specially true for making changes on the fly like the above
example. For `set_config` I would expect a more unified API, where actions in the `conf` object can be done like:

from pecan import conf

conf.set_config('some key', 'some value', overwrite=True)

--

I understand that there is a chance for collisions in the conf object with keys from a configuration file, but maybe something like:

    conf.pecan.set_config