Completions should play nicer with existing text

[alcarney]

Say I have an existing portion of a filename I want to complete

.. figure:: /path/to/example-

I position the cursor at the end of example- and hit Ctrl+Space and pick the item I want example-filename.png

.. figure:: /path/to/example-example-filename.png

The completion item pays no attention at all to the existing text and often requires some cleanup. The server should be taking existing text into account when generating completions. I imagine this issue is not specific to just filenames