added list instance query params (#197)

listOptions.go

@@ -1,7 +1,15 @@
 package govultr
 
-// ListOptions are the available fields that can be used with pagination
+// ListOptions are the available query params
 type ListOptions struct {
+	// These query params are used for all list calls that support pagination
 	PerPage int    `url:"per_page,omitempty"`
 	Cursor  string `url:"cursor,omitempty"`
+
+	// These three query params are currently used for the list instance call
+	// These may be extended to other list calls
+	// https://www.vultr.com/api/#operation/list-instances
+	MainIP string `url:"main_ip,omitempty"`
+	Label  string `url:"label,omitempty"`
+	Tag    string `url:"tag,omitempty"`
 }