fix: checker should also watch change in json files

[hackape]

People using resolveJsonModule: true get at-loader error about their json export, because checker doesn't pick up changes in any .json files.

This patch add .json as a valid file extension to be considered changed.

background: https://stackoverflow.com/questions/54695319/awesome-typescript-loader-does-not-pick-up-changes-in-json

[jkytomak]

Any chances to get this in?