Add some tests about numberFormats

[PeterAlfredLee]

Some users may not be familiar with numberFormats. See #481

This PR consists of some tests about minimumSignificantDigits, maximumSignificantDigits and notation. I'm trying to show some examples about the usage of these configurations. Hoping this can make it more clear.

[kazupon]

Thank you for your feedback!

I think that issue was opened because the docs example for NumberFormat was not good docs.
So, we should be added more example cases to NumberFormat section on vue-i18n docs.

What do you think about my comment?

[PeterAlfredLee]

So, we should be added more example cases to NumberFormat section on vue-i18n docs.

Agreed. I just pushed a new commit about it.
Feel free to let me know if there are something else to be modified. :-)

[kazupon]

Thank you!
You're always so reliable. :)

[PeterAlfredLee]

You're always so reliable. :)

I'm glad this helps a little bit. :-)