Add Signature Validation

[mathdroid]

According to the docs, valid requests from LINE platforms are the ones with X-Line-Signature header matching their digest value in request body.

https://devdocs.line.me/en/#signature-validation

[mathdroid]

One way to do it is by adding a verify function property to the bodyParser.json()'s first param. Then we will initialize the webhook conditionally (whether it uses the verification middleware or not) based on a verifySignature option.