Make the fields of Telegram types really optional (right now LinkPreviewOptions, ReplyParameters only)

[uralm1]

This PR is an example approach how to make optional fields of a secondary type classes really optional. I wrapped optional fields with boost::optional, right now only in LinkPreviewOptions, ReplyParameters classes.

It solves the problem described in issue #308

There are many such classes in Api, so consider this change as an example approach. Any way, it covers most used sendMessage() function, and solves problem with replies.

[Royna2544]

Code becomes really complicated, tho, wrapping everything in boost optional? Check my fork, can you test it? https://github.com/Royna2544/tgbot-cpp

[uralm1]

Code becomes really complicated, tho, wrapping everything in boost optional? Check my fork, can you test it? https://github.com/Royna2544/tgbot-cpp

jsoncpp is promising. I have some notes out of this topic while adapting my bot code, can you open issues in your project?

[Royna2544]

sure, i didnt know it was closed, now it is open