Allow optional lock reason

[nbartels]

In the github API, there is a lock reason defined.

I suggest to add this as a optional parameter. https://developer.github.com/v3/issues/#lock-an-issue

[presidentbeef]

Thank you for pointing this out, @nbartels. I don't think that was part of the API when I originally wrote the code.

I don't have a burning need for setting a reason, so if anyone else would like to open a PR for it, please be my guest.