epoch for luakit versions

[Markus Demleitner <msdemlei@ari.uni-heidelberg.de>]

Hi, 

For the luakit package that I'm trying to get back into Debian, I'd
like to use an epoch in the version.

Here's the story: Originally (and still reflected on
http://luakit.org), luakit used a date-based versioning scheme.  In
that, the last released version was 2017-08-10, which is what's
currently being packaged.

Meanwhile (and that's not yet reflected on luakit.org), there are
releases on Github (https://github.com/luakit/luakit/releases) that
follow a major.minor scheme, and the upstream maintainer has
confirmed they want to keep that scheme in the future.  

As we bring these newer releases into Debian, we would like to
reflect their version numbers in the Debian version number, too.

Opinions?

          -- Markus