Add implicit attribute: file.wordcount

[chrisgrieser]

Is your feature request related to a problem? Please describe.
The currently available wordcount plugins only count word in individual notes. For many usecases, e.g. longform writing where you split your project across multiple notes, counting words for many notes together would be an essential feature.

Describe the solution you'd like
An implicit attribute file.wordcount to easily display the word count. Basically what file.size is already doing, but in words as opposed to bytes/characters.

Describe alternatives you've considered
The longform plugin does no have such a feature yet. The wordcount plugins do not offer such a feature yet. I also considered writing a small shell script with wc and prepending the result to every file as yaml-header. However, such a solution would be impractical, as it creates yaml headers where you actually often do not want them (longform's compile does not address yaml headers yet, e.g.).