Add configurations which include all minor releases

[dominykas]

Some folks would likely want to have a larger test matrix based on their version support needs, at the cost of slower builds / more build minutes.

As suggested in #5, this could be done as a sub-folder inside a strategy folder, or it can be done as a suffix, e.g. gte-N-minors.yml.

This could somewhat address the "people should test in N.0.0" case (for packages that declare their engines support as >= N / N.x), to avoid accidentally breaking downstream users by using features available only in the latest releases (points raised in #6 (comment) and the remaining discussion).

[ljharb]

I already use this on https://github.com/ljharb/travis-ci/tree/master/node/minors in most of my packages; it seems useful.