Add npm tags to track major branches (1.x, 2.x, 3.x, etc.)

[jacobq]

Rather than just having latest and beta, it would be nice to have those for each mainline series, e.g.

• latest -> latest-4.x
• latest-4.x -> 4.0.1
• latest-3.x -> latest-3.1.x
• latest-3.1.x -> 3.1.1
• latest-3.0.x -> 3.0.14
• latest-2.x -> latest-2.0.x
• latest-2.0.x -> 2.0.16
• latest-1.x -> 1.8.8

and

• beta -> beta-4.x
• beta-4.x -> 4.0.0-beta.11
• beta-3.x -> beta-3.1.x
• beta-3.1.x -> 3.1.0-beta.5
• beta-3.0.x -> 3.0.0-beta.13
• beta-2.x -> beta-2.0.x
• beta-2.0.x -> 2.0.0-beta.8
• beta-1.x -> 1.8.2-beta.5

Perhaps this would make sense to set up with #55

[jacobq]

Closing (for same reasons as #77)