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_Examples.Output.verified.txt

@@ -0,0 +1,24 @@
+DESCRIPTION:
+The dog command.
+
+USAGE:
+    myapp <AGE> [LEGS] [OPTIONS]
+
+EXAMPLES:
+    myapp --name Rufus --age 12 --good-boy
+    myapp --name Luna
+    myapp --name Charlie
+    myapp --name Bella
+    myapp --name Daisy
+    myapp --name Milo
+
+ARGUMENTS:
+    <AGE>
+    [LEGS]    The number of legs
+
+OPTIONS:
+    -h, --help            Prints help information
+    -v, --version         Prints version information
+    -a, --alive           Indicates whether or not the animal is alive
+    -n, --name <VALUE>
+    -g, --good-boy