provide a post-build means to set messages sticky outside of config/environment file

[seemajune]

this is more of a feature suggestion than a bug:

our team is looking to set an option for users to be able to toggle auto-dismiss/sticky messages. This enables us to meet accessibility standards for WCAG-AA Success Criterion 4.1.3.

Currently, with every call we make to ember-cli-flash, we have to also pass sticky: isStickySetOnForUser. It would be great to have a place to set this based on user preferences, post-build rather than in the environment file.

thank you for your time <3
Seema