Add support for Spinners

[tjdavey]

Adds support for the spinners components introduced in Bootstrap 4.2.

Changes:

• Adds new Spinner component
• Add tests for new component
• Creates new page in documentation for new component
• Upgrades documentation CSS to Bootstrap 4.2 (given the claimed 4.2 support in the documentation itself)

Notes:

• I wasn't sure about the terminology for the property that switches between the two styles of spinner component. I couldn't think of a good term that wouldn't overlap with an existing property. animation was chosen at it was the most representative of the changes the property made, but I feel it may still be too ambiguous. I'm open to alternative suggestions.
• Whilst the documentation claims compatibility with Bootstrap 4.2 it was using a Bootstrap 4.1 CSS file. I've upgraded the documentation to a Bootstrap 4.2 version of the CSS file. I can remove this change and submit seperately if necessary.

[taion]

cool! can we add a type signature as well, for the TS users?

[tjdavey]

cool! can we add a type signature as well, for the TS users?

I'll add it now.

[taion]

Thanks!

[jquense]

this is awesome

[mxschmitt]

Super duper fast, good Job!

[tjdavey]

@taion I've added the Typescript support. I don't have the deepest understanding of the way the Typescript definitions are structured for this project so it might be worth just checking the details of my implementation.

[davecarlson]

Any news when beta 6 will be released so we can use these without pulling in master ?

:)

[jquense]

soon!