[RFE] Provide RHEL templates

[voxik]

It is quite annoying, that on RHEL, the default template is used. It would likely be better to provide either symlinks to relevant Fedora templates or their copies.

Since there might be more Ruby versions, there might also be advanced use cases such trying to guess the template based on the actual version of Ruby executing gem2rpm.

I am opening this RFE mainly in the context of RHEL documentation, where it is otherwise quite hard to explain what is the right thing to do.