Day period API

[akx]

This PR adds full support for day periods other than am/pm.

Requested by @sachinpali146 for additional time format character support. :)

See: http://unicode.org/reports/tr35/tr35-dates.html#Day_Period_Rules

[codecov-io]

Current coverage is 89.71%

Merging #349 into master will increase coverage by +0.01% as of 910f20c

@@            master    #349   diff @@
======================================
  Files           23      23       
  Stmts         3808    3840    +32
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit           3416    3445    +29
  Partial          0       0       
- Missed         392     395     +3

Review entire Coverage Diff as of 910f20c

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[RauliL]

LGTM.

[akx]

(@RauliL is a colleague who I recruited to do code review here.)