Support decode datetime

[jeffhsta]

Linked to #174

To use the datetime decode feature is just need to request it in the options, like:

%{now: Time.utc_now()}
|> Poison.encode!()
|> Poison.decode!(format_datetime: :time)

# %{"now" => ~T[02:24:23.871934]}

%{now: Date.utc_today()}
|> Poison.encode!()
|> Poison.decode!(format_datetime: :date)

# %{"now" => ~D[2018-10-11]}

%{now: DateTime.utc_now()}
|> Poison.encode!()
|> Poison.decode!(format_datetime: :datetime)

# %{"now" => #DateTime<2018-10-11 02:25:05.251816Z>}

%{now: NaiveDateTime.utc_now()}
|> Poison.encode!()
|> Poison.decode!(format_datetime: :naive_datetime)

# %{"now" => ~N[2018-10-11 02:25:13.460277]}

[coveralls]

Pull Request Test Coverage Report for Build 245

• 21 of 27 (77.78%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.03%) to 88.393%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/poison/parser.ex	21	27	77.78%

Totals
Change from base Build 236:	-1.03%
Covered Lines:	198
Relevant Lines:	224

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💛 - Coveralls