Format tag: Document attribute count

[eliot-akira]

Mentioned in:

• Format slug incorrectly converting accented characters template-system#118 (comment)
• How to limit maximum characters with ‘…’ after

It counts the number of items in a list, and also supports counting characters in a string (text).

We keep coming back to this topic of custom excerpts, so I understand there's a need for a better way to do things. This looks like it could help in that direction, for checking if a string needs to be trimmed.

<Format count>abcdefg</Format> = 7