Don't double up underscores with -transform=snakecase

[arp242]

It would treat an existing underscore as a "word", so one underscore got transformed in to 3.

I run in to this semi-regularly when I copy something like a database schema to a struct:

type Sequence struct {
	sequence_id  int64
	server_id    int32
	data_type    string
	increment_by int64
	cache_size   int64
}

And this gets transformed to:

type Sequence struct {
	sequence_id  int64 `db:"sequence___id"`
	server_id    int32 `db:"server___id"`
	data_type    string `db:"data___type"`
	increment_by int64  `db:"increment___by"`
	cache_size   int64  `db:"cache___size"`
}

I could fix up the field names first, which needs to be done anyway, or use -transform keep, but this makes it do the right thing with the defaults with a minimal of friction, and I don't expect it to break anything for anyone.