Add Amazon support (#3)

This commit introduces the Amazon support with calls to the Product Advertising
API (PA API). For now, I was only able to use the "www.amazon.fr" marketplace.
I will add more marketplaces when my Amazon Associate accounts will be
validated.

Signed-off-by: Julien Riou <julien@riou.xyz>

parser.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"regexp"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// Parser interface to parse an external service and return a list of products
+type Parser interface {
+	Parse() ([]*Product, error)
+	String() string
+}
+
+// filterInclusive returns a list of products matching the include regex
+func filterInclusive(includeRegex *regexp.Regexp, products []*Product) []*Product {
+	var filtered []*Product
+	if includeRegex != nil {
+		for _, product := range products {
+			if includeRegex.MatchString(product.Name) {
+				log.Debugf("product %s included because it matches the include regex", product.Name)
+				filtered = append(filtered, product)
+			} else {
+				log.Debugf("product %s excluded because it does not match the include regex", product.Name)
+			}
+		}
+		return filtered
+	}
+	return products
+}
+
+// filterExclusive returns a list of products that don't match the exclude regex
+func filterExclusive(excludeRegex *regexp.Regexp, products []*Product) []*Product {
+	var filtered []*Product
+	if excludeRegex != nil {
+		for _, product := range products {
+			if excludeRegex.MatchString(product.Name) {
+				log.Debugf("product %s excluded because it matches the exclude regex", product.Name)
+			} else {
+				log.Debugf("product %s included because it does not match the exclude regex", product.Name)
+				filtered = append(filtered, product)
+			}
+		}
+		return filtered
+	}
+	return products
+}