Make currently-active breakpoints accessible on `metaQuery`

[bensmithett]

... so you don't need to harvest them out of an onBreakpointChange handler - you can just query metaQuery.activeBreakpoints at any time to see what the current state of the world is.

My use case is checking for specific breakpoints in React lifecycle methods some time after an onBreakpointChange handler is added, but before the window actually changes size again.