Fix error on Layer section and reduce information on one line per step.

Instead of printing out multiple lines for some steps in Layer section,
now its only printing one line while other informations can be found in
Layer details. This change also provides fix for index out of bounds
error when user scrolls through steps in Layer section and there exists
at least one step with multi-line commands.

dive/image/layer.go

@@ -2,6 +2,8 @@ package image
 
 import (
 	"fmt"
+	"strings"
+
 	"github.com/dustin/go-humanize"
 	"github.com/wagoodman/dive/dive/filetree"
 )
@@ -39,5 +41,5 @@ func (l *Layer) String() string {
 	}
 	return fmt.Sprintf(LayerFormat,
 		humanize.Bytes(l.Size),
-		l.Command)
+		strings.Split(l.Command, "\n")[0])
 }