New feature: Struct tags to skip or shadow-copy a field while cloning

Changes

• [NEW] Add new struct tags per discussion in #20
  • clone:"skip" or clone:"-": Skip a field so that the field will be a zero value after cloned.
  • clone:"shadowcopy": Copy a field by value (a.k.a. shadow copy).

Full Changelog: v1.6.0...v1.7.0