measurements/word_length naming

`measurements/word_length` appears to return the average number of words in the input string(s), rather than the (average) word length.
Word length measurements could be a helpful measurement to add, but this measurement doesn't so is a bit misnamed from what I expected.

Some useful, distinct measurements in this family would include:
- **sentence_length:** The average number of characters in the input and/or the average number of words in a sentence
- **word_length:** The average length of words in the input
- **word_count:** The number of words in the input

I propose:
- Renaming this one (perhaps to `sentence_length` or `string_length`), and considering also returning the number of characters (without tokenizing) as an additional option.
- Or else incorporating into the already-existing `word_count`, which would mean simply averaging what's already calculated there.