`chmod` the config file to rw for user only

Prevent other users from potentially stealing the token.
@saghul my concern here is that:

```
➜  ~  node -e 'require("fs").writeFileSync("foo", "bar", { mode: 0777 })'
➜  ~  ls -l foo
-rwxr-xr-x 1 maciej maciej 3 Jul 26 17:22 foo
➜  ~  node -e 'require("fs").writeFileSync("foo", "baz", { mode: 0600 })'
➜  ~  ls -l foo                                                          
-rwxr-xr-x 1 maciej maciej 3 Jul 26 17:22 foo
```

So if user has a configuration file already, using `mode` with `fs.writeFileSync` won't change it.