Support snake case in subword-mode

[hanryhu]

go-mode does not respect subwords in snake case.

E.g., if point is at the start of my_snake_variable, M-f results in point at the end of my, but M-f M-f does not result in point at the end of my_snake. Instead, it jumps to the end of the whole name. Also, M-b always goes to the start of the whole name.

I suspect it's related to:
https://github.com/dominikh/go-mode.el/blame/166dfb1e090233c4609a50c2ec9f57f113c1da72/go-mode.el#L403-L404