Add delete directive

[queimadus]

Introduces a $delete directive which allows deleting a config sub-tree.
This proves useful for situations when there is a exception to the defaults where we need to remove one key.

[coveralls]

Pull Request Test Coverage Report for Build 120

• 36 of 36 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 99.836%

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Totals
Change from base Build 114:	0.006%
Covered Lines:	610
Relevant Lines:	611

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💛 - Coveralls