Show help inline, instead of using man

[scribu]

Benefits:

• no need to re-compile man pages after each change
• no need to have ronn installed
• works on Windows (fixes man command is not supported on Windows #546)

Tasks:

• prettify markdown
• reconcile wp and wp help invocations (fixes Consider including PHPdoc descriptions in CLI output #403)
• do something about _sys
• deprecate the $dest_dir parameter from WP_CLI::add_man_dir() (introduced in Where's the best place to locate a plugin's man file? #200)
• remove man/ folder

Related: #395, #108.

[danielbachhuber]

Holy whitespace batman.

[scribu]

Too much? Screenshot?

[danielbachhuber]

wp, which used to give me at least the last several commands, now just provides arguments:

Some pagination treatment here would work.

[scribu]

Yeah, that's what #552 is for.