Skip ignored private fields

[rafaelhl]

This pull request allows adding some private fields on properties structs without having problems, only marking this field as skipped with a specific tag as the test example below.:

	type S struct {
		p     string `properties:"-"`
		X     string `properties:"-"`
		Undef string `properties:",default=some value"`
	}

This change will avoid an error cannot set p using the struct above.

[rafaelhl]

@magiconair Can you approve this PR? This change will turn more flexible the usage of the struct when we need to add some other internal fields.