Omit the organization prefs dashboard ID if unset (#131)

* Add `WithOrgID` function to Grafana client
Issue: grafana/terraform-provider-grafana#747
This will allow the Terraform provider to use a different Org ID for each resource.
Currently, the client is configured once for each `provider` block

One caveat is that this feature can only be used with basic auth since API keys are org-scoped

* Fix linting

* Omit the organization prefs dashboard ID if unset
Will be used for grafana/terraform-provider-grafana#768

Author: julienduchesne

preferences.go

@@ -21,7 +21,7 @@ type QueryHistoryPreference struct {
 // Preferences represents Grafana preferences.
 type Preferences struct {
 	Theme            string                 `json:"theme"`
-	HomeDashboardID  int64                  `json:"homeDashboardId"`
+	HomeDashboardID  int64                  `json:"homeDashboardId,omitempty"`
 	HomeDashboardUID string                 `json:"homeDashboardUID,omitempty"`
 	Timezone         string                 `json:"timezone,omitempty"`
 	WeekStart        string                 `json:"weekStart,omitempty"`