ENHANCE: Make Config a singleton

[zacbrannelly]

Example:

from surround import has_config

@has_config
def some_function(config=None):
    config.read_from_dict({"override": False})

@has_config(name="local_config")
def some_other_function(local_config=None):
    assert(local_config["override"]) == False)

Also can be used like this if preferred:

config = Config.instance()

Can also be used to override:

@has_config(filename="override.yaml")
def some_function(config=None):
    ...

[ucokzeko]

@zacbrannelly Can you rebase this please? Thanks

[ucokzeko]

Just a recap from discussion with @zacbrannelly
We will be trying to add an option to load other config files as an optional parameter of @has_config

[ucokzeko]

LGTM
Good work @zacbrannelly!