generate bash completions unambiguously for fourth-level subcommands
complete with filenames for commands that have <file> in their synopsis
update Behat tests for completions
complete with command flags and associative parameters
remove conversion from 'wp --completions' to 'wp cli completions'

the format is different and associative parameters are required
Merge pull request #212 from wp-cli/completions-overhaul

Improve bash completions

fixes #547
fixes #994