Create a post build hook in config
Create a post build hook in config

This adds an optional attribute in the config
to execute a user-defined command at the end of
each build.
The format is as follows:

config {
   ...
   postBuildCmd: {
      exec: 'cmd',
      args: [ 'arg1, 'arg2', ... ]
   }
}

Fixes #17

google/dana