Character count's character/word count functions should be customisable

[36degrees]

Following on from #1104, whatever 'default' way we use to count characters (code points, bytes, characters) ideally it should match the way that a service's backend validation will count them. For this reason, we should consider making the count function customisable.

For example,

new CharacterCount($characterCount).init({ countFunction: string => string.length })

[fofr]

Same is probably true for word count: alphagov/govuk-design-system-backlog#67 (comment)

[36degrees]

Thinking about it, we might also want to support passing a regex as a string, as well as passing a function.