Make PR assignees + issue messages configurable

[tesseralis]

Hi! We'd like to use che-tsumi to maintain the various translations of the React documentation. I've tested out the bot and the main issue seems to be that some strings are hard-coded:

• the assignReviewers are assigned to vue-jp group (https://github.com/vuejs-jp/che-tsumi/blob/master/index.js#L140)
• the body of the issue created is in Japanese (https://github.com/vuejs-jp/che-tsumi/blob/master/index.js#L76)

It would be great if these can be assigned in an .env file (e.g. ASSIGNED_REVIEWERS).

This bot is great and we hope it gets more use in the React and Vue communities!

[re-fort]

Hi, I’ｍ the owner of this repository originally(it has transferred to organization repository) and happy to hear that!
We're willing to corporate with React team :)

IMO, it also would be good to set labels referred on .env file.
https://github.com/vuejs-jp/che-tsumi/blob/master/index.js#L77