Support installing the same versions as the snaps

[ali1234]

Is your feature request related to a problem? Please describe.
I use several Jetbrains IDEs such as CLion and Pycharm. If I stop paying for them I will need to use a fallback license, which requires specific versions of the software. Unfortunately unsnap does not seem to understand this, and will simply replace the snaps with the latest versions which I don't have a license for.

Describe the solution you'd like
unsnap should check the installed snap versions, and then install the same versions from flatpak when the snap version is held back.

Describe alternatives you've considered
Continuing to use snaps.

Additional context
This is apparently how you install a specific version.