add postbuild: step

Sometimes it can be useful to run some tools (e.g. zipalign) after the build (e.g. to make it reproducible).

I suggest adding a postbuild: entry to the YAML, analogous to the existing prebuild:.

Would probably also be nice if there was an easy way to get the path of the output APK in this postbuild: script, e.g. using $$APK$$.