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/Branch.Output.verified.txt

@@ -0,0 +1,18 @@
+DESCRIPTION:
+Contains settings for a cat.
+
+USAGE:
+    myapp cat [LEGS] [OPTIONS] <COMMAND>
+
+ARGUMENTS:
+    [LEGS]    The number of legs
+
+OPTIONS:
+                             DEFAULT
+    -h, --help                          Prints help information
+    -a, --alive                         Indicates whether or not the animal is alive
+    -n, --name <VALUE>
+        --agility <VALUE>    10         The agility between 0 and 100
+
+COMMANDS:
+    lion <TEETH>    The lion command