Add parsing life cycle and hooks

docs/parsing-and-hooks.md

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+# Parsing life cycle and hooks
+
+The processing starts with an array of args. Each command processes and removes the options it understands, and passes the remaining args to the next subcommand.
+The final command calls the action handler.
+
+Starting with top-level command (program):
+
+- parse options: parse recognised options (for this command) and remove from args
+- parse env: look for environment variables (for this command)
+- process implied: set any implied option values (for this command)
+- if the first arg is a subcommand
+    - call `preSubcommand` hook
+    - pass remaining arguments to subcommand, and process same way
+
+Once reach final (leaf) command:
+
+  - check for missing mandatory options
+  - check for conflicting options
+  - check for unknown options
+  - process remaining args as command-arguments
+  - call `preAction` hook
+  - call action handler
+  - call `postAction` hook