Add some minor improvements. (#271)

* Added an ImportConfig function which will accept a parsed configuration
file as a struct.
* Added a SetHTTPTransport function which will enable setting a HTTP
transport for tweaking low-level HTTP functionality.
* Added a SetCookieJar function which enables using a custom cookie jar
implementation.

Author: arangamani

goinsta.go

@@ -77,6 +77,12 @@ func (inst *Instagram) SetHTTPClient(client *http.Client) {
 	inst.c = client
 }
 
+// SetHTTPTransport sets http transport. This further allows users to tweak the underlying
+// low level transport for adding additional fucntionalities.
+func (inst *Instagram) SetHTTPTransport(transport http.RoundTripper) {
+	inst.c.Transport = transport
+}
+
 // SetDeviceID sets device id
 func (inst *Instagram) SetDeviceID(id string) {
 	inst.dID = id
@@ -92,6 +98,20 @@ func (inst *Instagram) SetPhoneID(id string) {
 	inst.pid = id
 }
 
+// SetCookieJar sets the Cookie Jar. This further allows to use a custom implementation
+// of a cookie jar which may be backed by a different data store such as redis.
+func (inst *Instagram) SetCookieJar(jar http.CookieJar) error {
+	url, err := neturl.Parse(goInstaAPIUrl)
+	if err != nil {
+		return err
+	}
+	// First grab the cookies from the existing jar and we'll put it in the new jar.
+	cookies := inst.c.Jar.Cookies(url)
+	inst.c.Jar = jar
+	inst.c.Jar.SetCookies(url, cookies)
+	return nil
+}
+
 // New creates Instagram structure
 func New(username, password string) *Instagram {
 	// this call never returns error
@@ -209,21 +229,28 @@ func Export(inst *Instagram, writer io.Writer) error {
 //
 // This function does not set proxy automatically. Use SetProxy after this call.
 func ImportReader(r io.Reader) (*Instagram, error) {
-	url, err := neturl.Parse(goInstaAPIUrl)
+	bytes, err := ioutil.ReadAll(r)
 	if err != nil {
 		return nil, err
 	}
 
-	bytes, err := ioutil.ReadAll(r)
+	config := ConfigFile{}
+	err = json.Unmarshal(bytes, &config)
 	if err != nil {
 		return nil, err
 	}
+	return ImportConfig(config)
+}
 
-	config := ConfigFile{}
-	err = json.Unmarshal(bytes, &config)
+// ImportConfig imports instagram configuration from a configuration object.
+//
+// This function does not set proxy automatically. Use SetProxy after this call.
+func ImportConfig(config ConfigFile) (*Instagram, error) {
+	url, err := neturl.Parse(goInstaAPIUrl)
 	if err != nil {
 		return nil, err
 	}
+
 	inst := &Instagram{
 		user:      config.User,
 		dID:       config.DeviceID,