Prevent endless loop in tokenization

Closes #198

src/Spectre.Console/Cli/Internal/Parsing/CommandTreeTokenizer.cs

@@ -63,17 +63,24 @@ namespace Spectre.Console.Cli.Internal
                 }
 
                 var character = reader.Peek();
-                if (!char.IsWhiteSpace(character))
+
+                // Eat whitespace
+                if (char.IsWhiteSpace(character))
                 {
+                    reader.Consume();
+                    continue;
+                }
+
                 if (character == '-')
                 {
+                    // Option
                     tokens.AddRange(ScanOptions(context, reader));
                 }
                 else
                 {
+                    // Command or argument
                     tokens.Add(ScanString(context, reader));
                 }
-                }
 
                 // Flush remaining tokens
                 context.FlushRemaining();

src/Spectre.Console/Cli/Internal/TextBuffer.cs

@@ -51,6 +51,12 @@ namespace Spectre.Console.Cli.Internal
             return true;
         }
 
+        public void Consume()
+        {
+            EnsureNotAtEnd();
+            Read();
+        }
+
         public void Consume(char character)
         {
             EnsureNotAtEnd();