Extension methods for Option<T> (#722)

* add `Option` extension methods to provide equivalent functionality to the existing `ArgumentExtensions` 

* rename `ParseResult.Suggestions` -> `GetSuggestions`

* simplify `ExistingOnly` overloads and improve tests

Author: jonsequitur

src/System.CommandLine.Benchmarks/CommandLine/Perf_Suggestions.cs

@@ -69,7 +69,7 @@ public void Setup_FromParseResult()
         [Benchmark]
         public void SuggestionsFromParseResult()
         {
-            _testParseResult.Suggestions("--wrong".Length + 1).Consume(new Consumer());
+            _testParseResult.GetSuggestions("--wrong".Length + 1).Consume(new Consumer());
         }
     }
 }

src/System.CommandLine.Tests/ParseDiagramTests.cs

@@ -41,16 +41,9 @@ public void Parse_result_diagram_helps_explain_parse_operation()
         public void Parse_result_diagram_displays_unmatched_tokens()
         {
             var command = new Command("command")
-                                      {
-                                          new Option("-x")
-                                          {
-                                              Argument = new Argument
-                                                  {
-                                                      Arity = ArgumentArity.ExactlyOne
-                                                  }
-                                                  .FromAmong("arg1", "arg2", "arg3")
-                                          }
-                                      };
+            {
+                new Option<string>("-x").FromAmong("arg1", "arg2", "arg3")
+            };
 
             var result = command.Parse("command -x ar");