refactoring: make `main` function. remove global variable. use `memoize` class for cache.
refactoring: remove unused variables, remove `options` for each functions.
refactoring and minor bug fix
add options and features: output/input issues to/from local json file.
change json format.
* move `body` field to `issue.formatted` field.
* copy `issue.local_id` to new `id` field.