Add --impacted-changes option to kmidiff

Add --impacted-changes option to kmidiff

kmidiff displays leaf changes by default and and it does so by *not*
showing impacted interfaces.

This patch add a --impacted-changes option to kmidiff so that it shows
the interfaces that are impacted by each leaf change.  Thus, by
default, no impacted interface is shown.

	* doc/manuals/kmidiff.rst: Document the new --impacted-changes
	option.
	* tools/kmidif.cc (options::show_impacted_interfaces): Add new
	data member.
	(options::options): Initialize the new data member to false.
	(display_usage): Add a description string for the new
	--impacted-changes option.
	(parse_command_line): Parse the new --impacted-changes option.
	(set_diff_context): Update the 'show-impacted-interface' property
	accordingly.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>