Allow more customised formatting of read status column

[Dominic-DallOsto]

Fixes #40

It is now possible to format the content of the read status column as either:

• icon and text
• only text
• only icon

And it is now possible to show the extension icon instead of "Read Status" as the column header.

[github-actions[bot]]

:rocket: This ticket has been resolved in v1.4.0. See Release 1.4.0 for release notes.