Inherit formatter

[podkot]

Hi.

When using component based localization, you have to import custom formatter to all the components. This patch allows components to inherit root's formatter.

Live example: https://jsfiddle.net/xocaao6o/1/
Both lines should be 'custom formatted'

One test is failing in the branch on my local setup:

SUMMARY:
✔ 158 tests completed
✖ 1 test failed

FAILED TESTS:
  datetime format
    getDateTimeFormat / setDateTimeFormat
      ✖ should be worked
        HeadlessChrome 0.0.0 (Mac OS X 10.13.2)
      AssertionError:   # test/unit/datetime.test.js:50

        assert.equal(text.textContent, '2012/12/20 下午12:00')
                     |    |
                     |    "2012/12/20 上午7:00"
                     HTMLParagraphElement{__vue__:#Vue$3#}

but it looks not related to the diff.

[codecov-io]

Codecov Report

Merging #269 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #269      +/-   ##
==========================================
+ Coverage   96.09%   96.09%   +<.01%     
==========================================
  Files           9        9              
  Lines         614      615       +1     
==========================================
+ Hits          590      591       +1     
  Misses         24       24

Impacted Files	Coverage Δ
src/mixin.js	96.55% <100%> (+0.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c5189f9...357b403. Read the comment docs.

[kazupon]

Thank you for your PR! 😺

Coud you add the unit tests please?

[podkot]

Unit test added, review please