Add utc option, add Changelog

Hi! Again, I found myself in need of a feature I wanted to implement correctly. The code change is minimal, but the feature is probably not too easy to understand. I hope I nailed it in the readme:

-----
#### utc
Type: `Boolean`
Default: `false`

Per default, the created `date` value will obtain the computer's timezone and
therefore not have UTC midnight as its time and will be a few hours off instead.

For example, when your timezone is 8 hours ahead of UTC: Creating a date object
from the input `"2000-01-01"` will result in `"1999-12-31T16:00:00.000Z"`,
because when your computer has the time of 00:00:00 on Jan 1st 2000, UTC time is
still in 1999. This is technically correct, but may not be what you want.

If you want to have easy-to-compare date strings in your JSON, set `utc` to `true`
and you will get `"2000-01-01T00:00:00.000Z"` as expected.

-----

Also, as mentioned in Issue #6, I introduced a short changelog so it's easier to keep track of changes (who would have guessed? ;)).

Since this is a new feature and a (non-breaking) API change, I pushed the version number to 1.2.0 if that's ok for you? 2 new test cases were introduced that test the new option.