Central default config #14

Problem statement

Currently there are two config modes:

1. Use hardcoded built-in defaults
2. Per-repo clawk.mod

It would be nice to be able to customise the behaviour of (1).

Proposed solution

Allow users to optionally create ~/.config/clawk/clawk.mod which would act as the general default for any repos without a clawk.mod defined.

Alternatives considered

Creating a clawk.mod for every repo, but more often than not this ends up being the same file over & over (in my experience sofar).