Support for network-activating a plugin upon install

Usage `wp plugin install user-switching --network`
Change syntax from `--network` to `--activate-network` for clarity
Fix the semantics of where `activate-network` is handled
'wp plugin activate' doesn't have an --activate flag
avoid reusing $assoc_args when invoking the activate() subcommand
Merge pull request #1066 from wp-cli/network-install

Support for network-activating a plugin upon install