Support time.Time on form binding

[andreynering]

It's required to inform the format:

type T struct{
        Time time.Time `form:"a_time" time_format:"02/01/2006"`
}

The default location is time.Local. If a utc tag is given, localtion is time.UTC:

type T struct{
        Time time.Time `form:"a_time" time_format:"02/01/2006" utc:"1"`
}

If user input is blank (""), then form.Time.IsZero() will return true.

[codecov-io]

Codecov Report

Merging #801 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #801   +/-   ##
========================================
  Coverage    93.35%   93.35%           
========================================
  Files           16       16           
  Lines         1324     1324           
========================================
  Hits          1236     1236           
  Misses          78       78           
  Partials        10       10

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[appleboy]

@tboerger @javierprovecho help to review this.