feat(modal): Add vertically centered option

[bastienmoulia]

Fix #1124

Also rename the modal documentation custom class in options with the following options:

• windowClass: 'dark-modal'
• size: 'sm'
• size: 'lg'
• verticallyCentered: true

[sciotta]

I really need this!

[ThrownLemon]

I would be happy to test this in my project.
Is there an easy way to import this version/commit using npm?

[DamianKedzior]

+1 For vertically centered option which is build-in in Bootstrap: https://getbootstrap.com/docs/4.0/components/modal/#vertically-centered
It just requires adding a new class. I tried to achieve it by adding "modal-dialog-centered" class to "windowClass" option but it adds the class to element instead of first

element which has "modal-dialog" class applied.

Maybe "windowClass" should add these classes directly to

element instead of element.

[thylux]

Looking forward to use this too.

[wgraham17]

Any chance this can get reviewed and pulled in?

[pkozlowski-opensource]

Adding it to the next feature-release (1.1.0) milestone.

[pkozlowski-opensource]

Thnx so much for this PR @bastienmoulia - I've resolved conflicts and landed it as 00e79c4 .
I've also changed the option name to better align with the Bootstrap's class name