Show help inline, instead of using `man`

Benefits:

* no need to re-compile man pages after each change
* no need to have `ronn` installed
* works on Windows (fixes #546)

Tasks:

* [x] prettify markdown
* [x] reconcile `wp` and `wp help` invocations (fixes #403)
* [x] do something about `_sys`
* [x] deprecate the `$dest_dir` parameter from `WP_CLI::add_man_dir()` (introduced in #200)
* [x] remove `man/` folder

Related: #395, #108.