Clear cache when toggling device emulation mode

[nhardy]

Is your feature request related to a problem? Please describe.

Scroll bar width can change when toggling device emulation mode. (e.g. 17px to 0px, vice-versa)

Describe the solution you'd like

Listen to window resize events and automatically clear the cache, or provide a more clear interface to clear the cache (__cache feels too much like an internal that consumers shouldn't touch).

Describe alternatives you've considered

Consumers of the package requiring this functionality can listen to window resize events and manually delete scrollbarWidth.__cache;

[xobotyi]

Actually it is intended to be set to undefined/deleted by consumers when they need sbw recalculation.

I'll clarify it in readme later.