Make config initialization a little more terse
Make config initialization a little more terse

This is something I suggested to Henric in code review here:

  https://gerrit.causes.com/#/c/27012/2/twigg/lib/twigg/config.rb

Get rid of the somewhat ugly `if` conditional by teaching
`config_from_file` to do nothing when `path` is `nil`; as a bonus, this
allows us to simplify the other call sites as well.

Change-Id: I08c74d5ef6aa83bbe95f71d11f309f6388ba6873
Reviewed-on: https://gerrit.causes.com/27121
Reviewed-by: Greg Hurrell <79e2475f81a6317276bf7cbb3958b20d289b78df@causes.com>
Tested-by: Greg Hurrell <79e2475f81a6317276bf7cbb3958b20d289b78df@causes.com>

causes/twigg,causes/twigg