Allow easier customization of Drawer close button

[steppy452]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Other... Please describe:

Current behavior

Currently, the close button for Drawer (possibly also Dialog?) is an x in the header component. And in order to customize, we have to create a new custom header component and replicate close functionality.

Expected behavior

Drawer component can take in a closeButtonIcon which will allow us to override the default x

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Easier customization to match unique styles

Environment

Libs:
- react version: X.Y.Z
- reablocks version: X.Y.Z


Browser:
- [ ] Chrome (desktop) version XX
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: XX  
- Platform:  

Others: