Imports should be grouped by default

[mkpankov]

Automatic import now inserts a new use line for every import: so std::collections::HashMap and std::collections::BTreeMap are imported in potentially different places.

I'd like to see automatic imports grouping at least by last component, so std::collections::HashMap becomes std::collections::{HashMap, BTreeMap} when making second import.

Grouping all imports from same library in one use statement should be optional.