Suggestion to rename the prefs "page_count_mode_count_words" and "words_per_page"

[ghost]

Since not everybody is using words to account for pages it seems to me to specific to call the above mentioned prefs "page_count_mode_count_words" respectively "words_per_page". It would be better if we could specify it for example in the following way:

"enable_count_pages": true,
"page_count_mode_unit": "chars", // can be "words", "chars", chars_without_spaces"
"units_per_page": 2400,

[titoBouzout]

I would accept a pull request!