Allow custom help providers (#1259)

Allow custom help providers

* Version option will show in help even with a default command

* Reserve `-v` and `--version` as special Spectre.Console command line arguments (nb. breaking change for Spectre.Console users who have a default command with a settings class that uses either of these switches).

* Help writer correctly determines if trailing commands exist and whether to display them as optional or mandatory in the usage statement.

* Ability to control the number of indirect commands to display in the help text when the command itself doesn't have any examples of its own. Defaults to 5 (for backward compatibility) but can be set to any integer or zero to disable completely.

* Significant increase in unit test coverage for the help writer.

* Minor grammatical improvements to website documentation.

test/Spectre.Console.Cli.Tests/Expectations/Help/Default.Output.verified.txt

@@ -1,4 +1,4 @@
-DESCRIPTION:
+DESCRIPTION:
 The lion command.
 
 USAGE:
@@ -10,7 +10,8 @@ ARGUMENTS:
 
 OPTIONS:
                              DEFAULT
-    -h, --help                                   Prints help information
+    -h, --help                                   Prints help information
+    -v, --version                                Prints version information
     -a, --alive                                  Indicates whether or not the animal is alive
     -n, --name <VALUE>
         --agility <VALUE>    10                  The agility between 0 and 100