fix: override variables in struct by non-default values

[DmitriyMV]

#325 addressed the problem with Parse functions changing already set fields with default values, but created a problem when you can't change already set fields with non-default values.

For example this works correctly:

1. Initialize struct with default values in tags and set struct fields explicitly.
2. Loading empty config/empty env using Parse.
3. Struct fields will remain the same if they were set during step 1.

But this does not:

1. Initialize struct with default values in tags and set struct fields explicitly.
2. Loading non-empty config or non-empty env using Parse.
3. Struct fields will remain the same if they were set during step 1 but should get the non-empty values from the step 2.

This PR fixes that.

[ccoVeille]

I would return a struct then

Suggested change

	func get(fieldParams FieldParams, opts Options) (val string, isDefault bool, err error) {
	type whatever struct {
	val string
	isDefault bool
	}
	func get(fieldParams FieldParams, opts Options) (whatever, err error) {

[DmitriyMV]

Done

[ccoVeille]

Maybe exists could be a field in the struct and getOr would simply return a gotValue.

Note: I'm not a maintainer, just a random reviewer. Maybe it's worth for someone else feedback

[DmitriyMV]

Any chance of this being merged?