Use CWWiFiClient.sharedWiFiClient().interface() to get current interface

'init(interfaceName:)' was deprecated in OS X version 10.10

fixes #12.

Merge pull request #10 from sekimura/exit-status

feat: set exit statuses

Merge pull request #6 from sekimura/no-wifi

feat: show a message when it has no wireless interfaces

doc: update the gif

feat: use current ssid with no args

chore: add the binary