Lazy calculate WordCount, ReadingTime and FuzzyWordCount

Lazy calculate  WordCount, ReadingTime and FuzzyWordCount

This avoids having to execute these expensive operations for sites not using these values.

This commit sums up a set of wordcounting and autosummary related performance improvements.

The effect of these kind of depends on what features your site use, but a benchmark from 4 Hugo sites in the wild shows promise:

```
benchmark           old ns/op       new ns/op       delta
BenchmarkHugo-4     21293005843     20032857342     -5.92%

benchmark           old allocs     new allocs     delta
BenchmarkHugo-4     65290922       65186032       -0.16%

benchmark           old bytes      new bytes      delta
BenchmarkHugo-4     9771213416     9681866464     -0.91%
```

Closes #2378