Add remove hashtag

### This pull request is "add `remove_hashtag`". 

Often hashtag is not necessary .

- retrobot.yml

```
remove_hashtag: true # (default=false / remove hashtag the tweet)
```

---

exp post

- before tweet

```
mirakui_retro hello #hashtag nospace#hashtag2
```

- after tweet

```
mirakui_retro hello nospace
```

Please check it ;)