Implement "omitalways" tag to allow decode yet prevent encode.

[Nifty255]

A developer may wish to decode or otherwise assign to a struct field but not re-encode. For example, a MongoDB document struct with a struct pointer inside for relationships:

type User struct {
  Email     string    `bson:"email"`
  Sessions  []Session `bson:"sessions,omitalways"`
}

This is accomplished via the following set of implementations:

• Implement flag in fieldInfo struct.
• Implement flag detection and assignment to fieldInfo struct.
• Implement skip-encode on flag true.
• Implement documentation and examples.