This is origonaly form this checkin.
I have only tried to cherry pick it on top of the current source
including the corresponding tests and fixtures.
You can now use a YAML list to add configuration files to be included
in the order of the list.
The syntax supports globs (to be used with `Dir[pattern]`), relative
paths to the config value of `source` and absolute paths.
Using this sample config:
or the glob version:
the new deep merge order is in both cases:
`Jekyll::DEFAULTS < _config < _config_a < _config_b`
I've added additional tests for three use cases:
* Having one inclusion
* Having a glob inclusion
* Having one inclusion with override in the main config