support alias as default source

src/isn/Settings.cs

@@ -1,4 +1,6 @@
+using System;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace isn
 {
@@ -14,6 +16,28 @@ namespace isn
         public Dictionary<string, SourceSettings> Sources { get; set; }
 
         public bool AutoUpdateApiKey { get; set; } = false;
-        public string DefaultSource { get; set; }
+
+        private string source;
+        /// <summary>
+        /// Default Url or alias for the source
+        /// </summary>
+        /// <value></value>
+        public string DefaultSource
+        {
+            get => source;
+            set
+            {
+                if (Sources.ContainsKey(value))
+                {
+                    source = value;
+                    return;
+                }
+                if (Sources.Values.Any(s => s.Alias == value))
+                {
+                    source = Sources[value].Alias;
+                }
+                throw new InvalidOperationException("Add it to Sources before, with an API key, as source or alias .");
+            }
+        }
     }
 }