Add :ExchangeClear command

Thanks for your great work. Really nice plugin this is!

In my case, I often clear "exchange" mode when edited something.
I want to realize it by defining the following autocmd.
```vim
autocmd TextChanged * ExchangeClear
```
How about this?

(Yes, I know `normal cxx` has the same effect but above one is better becase of robustness for re-mapping)
Taking cues from other plugins, I'd call this tag `*exchange-:ExchangeClear*`