deepStringAssign created and tested, closes #7

[fidel-ml]

The function deepStringAssign was created.
It uses the Utility Type Subset, that was also created, following a post in a blog (see documentation).

Subset is like Partial, but it works on any level of the object, not just the first.
It is needed by deepStringAssign so the elements used as sources for the changes may be incomplete on any level.

The function is parametric, so the type of the target is preserved.
The sources of changes contains subsets of the keys of the target object, and only those are overwritten (if their values have type string).

Different from the Object.assign function, deepStringAssign works by cloning the target object, and rewriting the corresponding keys with the values from the sources. This work is needed for the function to be used on existing constants without altering them.
The function Object.assign can be used on an empty object, and all options can be assigned, but in that way, the type of the original object is lost (objects with any combination of keys may be created).
The function deepStringAssign uses a copy of the target object, so all its keys are preserved, and only those can be overwritten.

[alanrodas]

Docs must be regenerated in order to merge. This issue will be presented over and over when multiple pull-requests are performed, and thus, maybe we need a different way of serving the docs.