Variables in snapcraft.yaml?

[Dan Kegel]

I'm snapping an app that uses a library.  The library has its
major number in its package name and directories, as is customary.
I'd like to use the same snapcraft.yaml even when the major
number of the library changes (we build several versions all
the time), except perhaps for changing a variable.

Right now, snapcraft doesn't support that, so I have to generate
the snapcraft.yaml file, which is kinda ugly.

Thoughts?