Transform placeables (escape values)

[slavafomin]

Hello!

Thank you for this great library!

However, I'm building HTML messages with Fluent and I need to insert user-generated content into the final messages using placeables. The thing is that user could provide values containing HTML, which could break the final markup or could even be dangerous. All templating systems allow to escape values passed to the templates. It would be great if Fluent would have such functionality as well.

E.g.:

new FluentBundle('en', {
  transformPlaceable: value => escapeHtml(value),
});

Fluent bundle has a transform option, but it receives everything, but not the values.

I know I can escape the values before passing them to Fluent, but having it at the Fluent level will be more convenient and will better mimic how templating systems work.

What do you think? Thanks!