Support self-upgrade

[jarun]

This is a ToDo list item to support in-place upgrade of the program.

[jarun]

If Buku is installed using a package manager, this option may trump the version provided by the package manager. Upgrading asynchronously may lead to inconsistencies and unforeseen consequences.

As an alternative, option --upstream is provided to check the latest version available upstream. The relevant commit is 1d1f7a9.

Upgrading downstream packages to the latest upstream release takes a while due to diverse processes involved in package management. Generally it's a matter of a few days. While it's possible to download and use the latest version manually, we strongly suggest users to follow a single mechanism to install and upgrade a program.

Closing the issue with the above information.