feat(config): support MERGE-PRIORITIES for Config items

[ankostis]

(I hope you don't get too mad at me for trying...)

Retrofitted loader.Config class to support merging-priorities for specific values based on ranks (e.g. integers). Now update_config() always respects priorities. The purpose it to use it to fully support also #99 environment-variable priorities.

For this PR, the new features are used also to deprecate the need to re-update Application.cli_config on top of file-configs.

Implementation

• A lot of effort was put to have code and the memory-footprint as small as possible.
  • Config instances by default don't waste extra memory - default priority resolved from _merge_priority class attribute.
  • Possible to set a single rank(number) that propagates to all config-object values; these apply to 1st level only. But it can be set recursively to inner config-objects (wasting an in per config-object).
  • To support different ranks in the same config-object, the _merge_priority is upgraded to dict.
  • The original idea in Envvars #439 of storing tags (a new dict) into config ibjects would be an overkill.
  • Have to study performance-tax on the merges.
• A new Config.substract() method is used to limit value-changes, taking into account priority-ranks.
  This is useful to preserve traits that have been directly modified between loading of configs from different sources.
• The ranks must be comparable. besides that, the code does not enforce any other limitation on their type APART from setting the default Config._merge_priority = 0 which in effect "force" the use of integers.

[minrk]

I appreciate the effort, but as I described in #439, I think we should not introduce the concept of priority to config objects.

[ankostis]

For whoever lands here, latest changes for the same functionality in ankostis@confranks.