keep Netmask map results around, instead of re-computing each time

[newhouse]

It seemed unnecessary / inefficient to build up the blocks array during each call to ipv4_check.

This change builds the Netmask array map, and keeps it around for re-use during each ipv4_check call.

[frenchbread]

Thanks for the PR, @newhouse.

Merging.

[newhouse]

You're welcome!

Any idea when you'll publish a new version with this change? I'd like to update my package...

[frenchbread]

@newhouse, v2.2.1 version has already been published and should be available via npm