Be smarter about available versions (& support alpha builds)

[adam-lynch]

Depends on nwjs/nw.js#4037.

This module would then get and validate versions a lot more solidly.

• All versions would be grabbed reliably.
• It would be a lot easier to avoid alpha builds when the version option is defaulted to latest.
• Changes to the platform ZIPs between versions won't break anything.
• People could use alpha builds if they like.
• If the user tries to build for a version which doesn't support that platform, an error will be thrown.

Fixes #239, #253, #254, (probably) #255, #259, #262, and probably more.

[gaving]

A big 👍 for this from somebody constantly working off alpha builds!

[jvkassi]

update ? nwjs/nw.js#4037 is kinda fix now, right ?

[adam-lynch]

Closed as of 2.2.1 (#337)