Add support for setting ID Token nonce from Unity

[onevcat]

This should solve #56. In this PR we added the nonce property to the login option, to allow Unity users to set a customized nonce for verifying against their server. To use it, set the IDTokenNonce value in the option and pass it to Login:

var scopes = new string[] {"profile openid"};
var option = new LoginOption();
option.IDTokenNonce = "abc123";

LineSDK.Instance.Login(scopes, option, result => {
     // handle the result
});

[onevcat]

@plateaukao This time I also tried to implement the Android side too. Please check and review it when you have time! Thanks.

[daviscdev]

It seems like a typo.
Is it correct that the space is removed intentionally?

[onevcat]

Thanks! Should be fixed in 835b3a7

[daviscdev]

LGTM. Thank you!