Carbon coverage: Address storage issues

[RichKummer]

Currently, the functional prototype for the Carbon coverage plugin does not have storage that persists once the user has closed the plugin and re-opened it. This means that when the user would hide or ignore a particular issue, it would reappear if the plugin were closed and then re-run.

Ideally, an issue that a user flags as "hidden" should remain hidden between sessions. Otherwise, this can lead to additional work on the part of the user to correct each time the plugin is run, and add to confusion using the plugin.