Add option to filter out non ascii chars

[braunbearded]

This will allow a user filter out non assci characters via --onlyascii flag

[edermi]

Hey @braunbearded, I tried to simplify your logic a little bit, you may have a look at it. I also swapped out your isASCII check with a library check provided by golang.org/x/exp/utf8string. Although your ASCII check didn't look wrong, when dealing with encodings, I think relying on a dedicated library is almost always a better approach compared to writing stuff on your own. There are too many corner cases and weird decisions you may not be aware of, therefore I hope this is fine for you.

I did some rudimentary checks, you may also have another look at it to make sure everything still works as it is supposed to.

[braunbearded]

Yeah using a library in this makes sense especially when dealing with encoding.

Thanks for the feedback!