Extend struct tags with custom type and format

[tcriess]

Allows for custom type and custom format in the "goas" struct tag. This is f.e. for custom "Null"-types that are supposed to behave like nullable variants of standard types.

type Model struct {
    Active NullBool `json:"active" goas:"type=boolean"`
    Created NullTime `json:"created" goas:"type=string,format=date-time"`
}

with

type NullBool struct {
    Valid bool
    Bool bool
}

(NullBool and NullTime have their respective json marshaler to behave like bool and time.Time)