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_Examples.Output.verified.txt

@@ -0,0 +1,30 @@
+DESCRIPTION:
+The animal command.
+
+USAGE:
+    myapp animal [LEGS] [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
+    myapp animal horse --name Cash
+    myapp animal horse --name Dakota
+    myapp animal horse --name Cisco
+    myapp animal horse --name Spirit
+
+ARGUMENTS:
+    [LEGS]    The number of legs
+
+OPTIONS:
+    -h, --help     Prints help information
+    -a, --alive    Indicates whether or not the animal is alive
+
+COMMANDS:
+    dog <AGE>    The dog command
+    horse        The horse command