pull in PRs from upstream

[goapunk]

possibly intersting:

• Improve performance of addLayers by preallocating array Leaflet/Leaflet.markercluster#988 (we already use this but it has been updated to fix a bug)
• Adding "markers" for clusters in aria-label for accessibility Leaflet/Leaflet.markercluster#1044
• Check for finite min and max zoom properties Leaflet/Leaflet.markercluster#991
• Avoid crashing with some zoom events Leaflet/Leaflet.markercluster#1048