[feat] Add a 'binary' flag

[darrachequesne]

So that the call to the has-binary method can be skipped. Usage:

// with binary data
socket.binary(true).emit("binary", obj);

// without binary data
socket.binary(false).emit("string", obj);

// call to hasBin
socket.emit("guess", obj);

Note: the socket.io.js file is the generated output of make socket.io.js, and should not be manually modified.

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

New behaviour

Other information (e.g. related issues)