option to disable cache expiration

[timlau]

https://bugzilla.redhat.com/show_bug.cgi?id=1272495

yumex-dnf allows the user to configure expiration of cache after 1-240 hours. There should also be an option to disable yumex-dnf control of the cache, such as by allowing the user to enter -1.

dnf allows for more fine-grained control of cache expiration (e.g. different expiration settings per repository -- particularly relevant since the fedora non-updates repo never changes), and some users would prefer to leave the cache control to dnf.

Use case: I have things set up outside of yumex-dnf so that dnf always keeps its cache up-to-date, yet with the default yumex-dnf settings, virtually every time I open yumex-dnf, it redownloads the repo data. This is a big deal for me because on my connection it takes 5-10 minutes to download the repo data. I have finally set yumex-dnf to only expire the cache every 10 days (the maximum value allowed), but this is still sub-optimal because yumex-dnf should never need to expire the cache with my setup.