SpinButton: Add getClassNames prop to allow complete customization of the component

[kysedate]

Pull request checklist

• Addresses an existing issue: Fixes #0000
• Include a change request file using $ npm run change

Description of changes

Adding a prop to the Spin Button that allows for a complete override of the fabric styles

[msftclas]

All committers have signed the CLA.

[dzearing]

rather nervous of this change. A better approach would be to avoid the getClassNames approach and use the 6.0 getStyles functional approach. We found that getClassNames required us to come up with every possible permutation of areas and states, whereas a function of state can produce a given set of area classes.

See the ghdocs/styling.md file for more description.

[christiango]

@dzearing this is another case where we need something that lets us completely override the styling in the 5.0 timeframe for Office Online.

[christiango]

Should we just move it to getStyles in the 5.0 branch?

[christiango]

@montselozanod can you work with @kysedate to see how we can unblock this work? Can we perhaps go straight to the new styling API safely?

[montselozanod]

@christiango sure!! I'll talk with @kysedate so we can go over this together