Add ToJSONReader, ToRSSReader and ToAtomReader

Author: p1ass

Diff for: README.md

@@ -11,7 +11,12 @@ go get -u github.com/p1ass/feeder
 
 ### Examples
 ```go
-import "github.com/p1ass/feeder"
+package main
+
+import (
+	"github.com/p1ass/feeder"
+	"time"
+)
 
 func crawl(){
 	rss1 := feeder.NewRSSCrawler("https://example.com/rss1")
@@ -30,9 +35,14 @@ func crawl(){
 		Items:       items,
 	}
 
-	json, err := feed.ToJSON() // json is a `string`
-	rss, err := feed.ToRSS() // rss is a `string`
-	atom, err := feed.ToAtom() // atom is a `string`
+	json, err := feed.ToJSON() // json is  string
+	rss, err := feed.ToRSS() // rss is string
+	atom, err := feed.ToAtom() // atom is string
+
+	jsonReader, err := feed.ToJSONReader() // jsonReader is a io.Reader
+	rssReader, err := feed.ToRSSReader() // jsonReader is a io.Reader
+	atomReader, err := feed.ToAtomReader() // jsonReader is a io.Reader
+
 }
 
 ```

Diff for: generator.go

@@ -1,22 +1,54 @@
 package feeder
 
-import "github.com/p1ass/feeds"
+import (
+	"io"
+	"strings"
+
+	"github.com/p1ass/feeds"
+)
 
 // ToRSS generates RSS feed
 func (f *Feed) ToRSS() (string, error) {
 	return f.convert().ToRss()
 }
 
+// ToRSSReader generates RSS feed and return io.Reader.
+func (f *Feed) ToRSSReader() (io.Reader, error) {
+	rss, err := f.convert().ToRss()
+	if err != nil {
+		return nil, err
+	}
+	return strings.NewReader(rss), nil
+}
+
 // ToAtom generates Atom feed
 func (f *Feed) ToAtom() (string, error) {
 	return f.convert().ToAtom()
 }
 
+// ToAtomReader generates Atom feed and return io.Reader.
+func (f *Feed) ToAtomReader() (io.Reader, error) {
+	atom, err := f.convert().ToAtom()
+	if err != nil {
+		return nil, err
+	}
+	return strings.NewReader(atom), nil
+}
+
 // ToJSON generates JSON feed
 func (f *Feed) ToJSON() (string, error) {
 	return f.convert().ToJSON()
 }
 
+// ToJSONReader generates JSON feed and return io.Reader.
+func (f *Feed) ToJSONReader() (io.Reader, error) {
+	json, err := f.convert().ToJSON()
+	if err != nil {
+		return nil, err
+	}
+	return strings.NewReader(json), nil
+}
+
 func (l *Link) convert() *feeds.Link {
 	return &feeds.Link{
 		l.Href,