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/Root_Examples_Leafs_Eight.Output.verified.txt

@@ -0,0 +1,19 @@
+USAGE:
+    myapp [OPTIONS] <COMMAND>
+
+EXAMPLES:
+    myapp animal dog --name Rufus --age 12 --good-boy
+    myapp animal dog --name Luna
+    myapp animal dog --name Charlie
+    myapp animal dog --name Bella
+    myapp animal dog --name Daisy
+    myapp animal dog --name Milo
+    myapp animal horse --name Brutus
+    myapp animal horse --name Sugar --IsAlive false
+
+OPTIONS:
+    -h, --help       Prints help information
+    -v, --version    Prints version information
+
+COMMANDS:
+    animal    The animal command