Support Immutable config object

[akalex]

It would be nice if I could have the possibility to set that the config object is immutable once I loaded all needed configuration files. In that case, I can be sure that my configuration won't be changed during runtime.
Just an example of it could be used

from pymlconf import Root, freeze

config = Root()
for yml_file in CONFIG_PATHS:
    if yml_file.exists():
        config.loadfile(str(yml_file))
freeze(config)

config.foo = "bar"

TypeError is raised