With the eventual intent on moving some `main.py` functionality into modules,
it will be useful to reuse some of core's logic for temporary cli use. Thus,
we will need to spin up temporary instances of `Halibot` to load config, load
modules, etc.

This adds an optional keyword `config` to `Halibot`, which sets a config dict
as supplied. By default, this value is empty, as it was previously.

This also adds an optional keyword `configfile` to `Halibot`, to specify an
alternate configuration file to load on `._load_config()`. Therefore, this
method has also been updated to accept an optional parameter `configfile`
parameter of its own, in the event some external code wants to load an
alternate code without running the full start process.

The default config file remains `config.json`. In theory, no changes to
other code is necessary.

**NOTE:** With this change, `config` is no longer a static class variable,
and is now purely an instance variable. This probably shouldn't affect
anything, but multiple `Halibot` objects will no longer have the same
config dict object.