'main': Support the non-precommand flags of sudo(8) and ssh-agent(1).

Uses the infrastructure added in the previous commit.

Fixes #678.

highlighters/main/test-data/precommand-killing2.zsh

@@ -35,5 +35,5 @@ BUFFER='sudo -e /does/not/exist'
 expected_region_highlight=(
   '1 4 precommand' # sudo
   '6 7 single-hyphen-option' # -e
-  '9 23 default "issue #678"' # /does/not/exist
+  '9 23 default' # /does/not/exist
 )