Add a new vim hook on CompleteDone. This hook is called when a
completions is selected.
When forcing semantic completion with the keybind, C# completions can
return a list of importable types. These types are from namespaces which
havn't been imported, and thus are not valid to use without also adding
their namespace's import statement. This change makes YCM automatically
insert the necessary using statement to import that namespace on
completion completion. In the case there are multiple possible
namespaces, it prompts you to choose one.
