Add a hook to allow other modules to alter the list of glossary terms

Glossify module is not compatible with cases, such as Entity Translation or the Synonyms module, where a glossary term has multiple variations. Built-in support for these modules would be great; or Glossify could provide a hook which allows other modules to alter the list of glossary terms.

This patch adds a glossify_terms_alter() hook for modules to modify the list of terms. Another module would have to be developed which implements the hook and looks up terms translated by Entity Translation, for example.