Allow break line between import groups

[kazuki43zoo]

Google Java Style Guide says as folows in "3.3 Import statements":

Imports are ordered as follows:

1. All static imports in a single block.
2. All non-static imports in a single block.

If there are both static and non-static imports, a single blank line separates the two blocks. There are no other blank lines between import statements.

But the current eclipse-java-google-style.xml not allow adding the break line between import groups.

[Marcono1234]

Partially fixes #273 (the main problem for #273 is that the formatter config apparently cannot specify the import groups, so the user would have to edit them manually, as shown in https://github.com/google/styleguide/issues/273#issuecomment-457843194)

See also #271