add a new option overwrite

[JounQin]

Enable to reload environment variables for nodemon, etc.

[maxbeatty]

this introduces some potentially dangerous behavior (which has been discussed ad nauseam in other issues). if nodemon spawns a new process and your code in the process calls dotenv, you should realize changed variables

[JounQin]

@maxbeatty But actually it can't be changed automatically with nodemon. Then I can only overwrite the environments with dotenv API, of course I can do it, but hope it could be better.