Fix an issue about bold/italic toggling with CJK

Before this change, we can only remove bold attribute of whole words.
```
This is a **bold** text
```
We select the word 'bold', click the 'bold' button and everything is okay.
```
This is a bold text
```

However, in Chinese/Japanese/Korean we don't have spaces between words.
```
No**spaces**between words in CJK
```
Therefore, selecting 'spaces' and clicking the 'bold' button will not work properly. It becomes
```
Nospaces**between words in CJK
```

I fixed this and it worked well for both Latin and CJK characters. Please take a look if possible. Thanks!