add build matrix to CI with optional features

[rursprung]

it seems that currently optional features (chrono & aq_unstable at this time) are not being tested by the CI workflow. i'd suggest adding a build matrix which covers also these features.

see e.g. what i did in rursprung/r2d2-oracle#15 for using a build matrix with features. with #58 i'm adding a build matrix to this repo here anyway, so maybe you could then directly extend that and just add the features to the matrix as well (so that they're also tested with both versions of rust and on all OS). you might want to test all four combinations (no features, all features and each feature individually) or just two of them: all features and no features.