Provide a drupal_alter() to allow modules to modify the extension -> content model map

[mjordan]

We should define a drupal_alter() to allow modules to add to or override the extension -> collection model map at https://github.com/MarcusBarnes/islandora_compound_batch/blob/master/includes/utilities.inc#L20. Potential use cases include ingesting custom content models and defining which extensions should be which extensions should be assigned the binary compound model.

Possible related issue is #16.