add support for merging multiple coverage profiles

[lyyyuna]

No description provided.

[CarlJi]

we already use such function like https://github.com/qiniu/goc/blob/master/pkg/cover/server.go#L174

	merged, err := cov.MergeMultipleProfiles(mergedProfiles)
	if err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}

Based on the function , we probably just need implement a subcommand like goc merge cov1 cov2 cov3 ... to do that.