Make it easy for users to add new data to glue once qglue has been launched

[astrofrog]

This may already be possible, but if so we need to document it - Harry Ferguson mentioned he would like to be able to do something like:

>>> app = qglue(data=...)
>>> app.add_data(more=...)

and have the new data appear in glue automatically.

[ChrisBeaumont]

Right now the only supported syntax is app.data_collection.append(...), but that doesn't do the same kinds of smart-parsing that qglue does. +1