This uses a fully custom function for indenting Go code in Vim.
It provides a lot more flexibility than a cindent-based approach,
so this version gets the := operator correct, as well as switch
labels and jump labels.

One outstanding TODO is to handle lines immediately after jump labels.

R=adg, n13m3y3r, jnwhiteh, dchest, rsc, rlight2
CC=golang-dev, rivercheng
https://golang.org/cl/4534047