Implement merging strategies

[frank-lenormand]

Related to #14.

It would be nice to be able to specify a merging strategy to guide Confight as to what its expected behaviour would be when merging two dictionaries.

Examples include:

• merging lists
• ignoring the right hand value if the left hand value isn’t set (i.e. only update, no upsert)
• replacing the entire left hand dictionary with an existing right hand dictionary (i.e. the default behaviour of {**left, **right})
• matcher objects that dynamically set a strategy, predicated on the current key/dictionary being processed