nchars()-method for character vector

In the example for the detect_duplicates,list-method, I have this snippet. Should be a method.

```r
chars <- table(tolower(strsplit(paste(unlist(x), collapse = ""), "")[[1]]))
chars <- chars[grep("[a-zA-Z]", names(chars))]
char <- names(chars[order(chars, decreasing = FALSE)][1:20])
```