fix: Provide more meaningful default version info

This change allows the version info to be inferred at build time to the best
extent possible.

If buildTime is omitted, it will default to the current time.
If the gitCommit is omitted, it will default to "unspecified", indicating that
this was not passed by an LD flag. The docs reflect how to properly set this
flag.

pkg/version/version.go

@@ -13,17 +13,21 @@ var versionFormat = "git commit: %s\nbuild date: %s"
 
 // GitCommit returns the git commit
 func GitCommit() string {
+	if gitCommit == "" {
+		gitCommit = "unspecified"
+	}
 	return gitCommit
 }
 
 // Time returns the build time
 func Time() *time.Time {
+	now := time.Now()
 	if len(buildTime) == 0 {
-		return nil
+		return &now
 	}
 	i, err := strconv.ParseInt(buildTime, 10, 64)
 	if err != nil {
-		return nil
+		return &now
 	}
 	t := time.Unix(i, 0)
 	return &t