Allow modules to alter the array of entity types and label property used to get the entity autocomplete title

Problem/Motivation

Currently this module only supports nodes, and had planned to allow other modules to support additional entities but hasn't yet.

Proposed resolution

Add an alter hook. The function name is a little long, but unless you want to change this we should follow that pattern.

Remaining tasks

Add alter hook and API docs.

User interface changes

None.

API changes

New alter hook.