Allow either string or procs to be used in formatting DateTime objects

[mcclayton]

This aims to address #120

Previously, we had a global and field level option datetime_format which takes in a string representing the strptime format to format the Date/DateTime object. This PR enables datetime_format to be either a String or a Proc. This enables more flexibility/control in how dates can be formatted. ( i.e. for those who want to have something like integer UNIX timestamps)

This is something that can _technically already be achieved (only at the field-level) _ by passing a block directly to the field, however, now that datetime_format can be set globally, this allows us to set a global datetime_format Proc which will get invoked on any field that responds to strptime, and matching field-level syntax in a more idiomatic way.

Global Config Setting

If a global datetime_format is set (either as a string format or a Proc), this option will be
invoked and used to format all fields that respond to strptime.

Blueprinter.configure do |config|
  config.datetime_format = ->(datetime) { datetime.nil? ? datetime : datetime.strftime("%s").to_i }
end

Field-level Setting

Usage (String Option):

class UserBlueprint < Blueprinter::Base
  identifier :name
  field :birthday, datetime_format: "%m/%d/%Y"
end

Output:

{
  "name": "John Doe",
  "birthday": "03/04/1994"
}

Usage (Proc Option):

class UserBlueprint < Blueprinter::Base
  identifier :name
  field :birthday, datetime_format: ->(datetime) { datetime.nil? ? datetime : datetime.strftime("%s").to_i }
end

Output:

{
  "name": "John Doe",
  "birthday": 762739200
}

[AllPurposeName]

Hmm the error here is based on us using a datetime formatter on an invalid field, correct? The error message we get reads 'Cannot format DateTime object with invalid formatter' The formatter in this test case is perfectly fine, no? I think we need an invalid field error or something.

[AllPurposeName]

Nevermind I was confused. The message DOES NOT read 'Cannot format DateTime object with invalid formatter', rather it's 'Cannot format invalid DateTime object' which is perfectly valid.

[philipqnguyen]

👍