Use cdvPluginPostBuildExtras instead of ext.postBuildExtras

[kbrownlees]

Description

Only one ext.postBuildExtras can exist, if two plugins both define this function then only one of them will be run.

Should be able to use cdvPluginPostBuildExtras.add({}) instead, eg: https://issues.apache.org/jira/browse/CB-14163.

[szh]

This would be fixed in #599