rename InternalAssoc to InternalFlags

'flags' is more descriptive than 'assoc'
move --man handling to _run_command() so that `wp db --man` works
reduce code duplication by merging show_help_early() into Help_Command
create empty help.txt file, so that `wp help --man` works
create empty home.txt file, so that `wp home --man` works
remove old db man pages
help: add test for --help
Merge pull request #460 from wp-cli/man-refactor

Improve man page generation