add config spec
add tests for --quiet flag and don't suppress output on fatal error
replace WP_CLI_QUIET with WP_CLI::get_config('quiet')
make use of defaults
move special args handling to separate class
rename bootstrap() to load_depdencies()
introduce WP_CLI\Runner class

It contains all the logic related to argument handling and WP loading.
move even more code from utils.php into WP_CLI\Runner
storing the config spec in a static variable is not needed

get_config_spec() is only called once per instantiation, or at most 2
times.
add --param-dump
Merge pull request #277 from wp-cli/config-spec

Better organization of config/runtime keys