Related to #997.

Is your feature request related to a problem? Please describe.
It is now possible to find out EPUB CFI of currently visible page by pressing Ctrl-L. Also user is able to copy selection to clipboard. Would be great to have ability to copy CFI of selected position or fragment to clipboard to be able to place a link to that position or fragment in another application like note-taking software.

This could also address #981.

Describe the solution you'd like
Option in right-click menu or selection popup. This can be also implemented in more generic way as giving possibility for user to add custom actions with access to current context (like book, position, selection etc).