The common extension allows for definition of global shortcuts.
Within the configuration file shortcuts may be defined. For example, the following is a portion for the configuration file used to generate this page. This defines the "libMesh" and "MOOSE" website shortcuts, which may be used as traditional markdown shortcuts, as shown in Example 1.
globals: type: MooseDocs.extensions.common shortcuts: libMesh: http://libmesh.github.io/ MOOSE: http://www.mooseframework.org