move Utils\set_user() to Runner::set_user()

it's not a general-purpose utility and it shouldn't be public
extract find_command_to_run() method
make load_command() and load_all_commands() standalone utilities
`$group` is optional for the function, so let it be optional for the command too
Regenerate cache man
Merge pull request #485 from wp-cli/cache-delete-optional

Allow <group> to be optional for wp cache delete
core config: fix notice when not passing --dbpass
core config: set WPLANG based on downloaded locale

fixes #488
bump version to 0.10.0-beta
Create `get_api_for_version()`, which will give us the proper download link for a given version
rename get_api_for_version() to alter_api_response() and use in plugin command
Merge pull request #463 from wp-cli/theme-version

theme install: --version parameter ignored
contributing: use `behat --expand`

Without --expand, it's hard to debug failures in scenarion outlines.
behat: generate new plugin instead of relying on Hello Dolly

This allows us to check more things, since all the plugin data is frozen.
Ensure `wp plugin delete` and `wp theme delete` output consistent messages.
option: add success messages

see #472
core is-installed: let the bootstrap process show the error message.
composer: update mustache to 2.3.x; update composer.lock
bump version to 0.10.0-beta2

[ci skip]
Only use maintenance mode if a plugin or theme needs updating

When updating plugins (wp plugin update-all) or themes (wp theme update-all) on a WordPress multisite installation, maintenance activated and deactivated even if there are no updates to install.

The Plugin_Upgrader::bulk_upgrade() and Theme_Upgrader::bulk_upgrade() WordPress functions currently don't check that the array passed to it isn't empty, causing maintenance mode to be enabled even if no updates are required.

This bug doesn't affect non multisite installs, because for non multisite installs the code in Plugin_Upgrader::bulk_upgrade() and Theme_Upgrader::bulk_upgrade() only uses maintenance mode if the plugin/theme being updated is activated.

Fixes #491