Customizable expansion keys

[native-api]

Support multiple env: (environment variables -- based) axes via a list of lists

This feature was requested in e.g. https://stackoverflow.com/questions/46575415/can-i-multiply-options-in-matrix-include-with-the-env-option , https://github.com/travis-ci/travis-ci/issues/1519 . The stumbling block seems to be that the root level of .travis.yml is a hash table, so it can’t have duplicate keys, nor can it contain list entries. However, YAML supports lists of lists: env: - - VAR1=foo - VAR1=bar - - VAR2=baz - VAR2=xyzzy and detecting a list of lists as opp…

Support a union of build matrices

Practice has shown that it’s impractical to have a single build matrix for multiple OSes because the environments and .yml parameters they support are too different and non-intersecting. I’m hereby suggesting to have multiple build matrices under e.g. matrix: union: that would be expanded independently. A side benefit would be an ability to have matrices for all build stages rather than just the default one.