Add --files-with-matches flag.

Closes #26.

Acts like --count but emits only the paths of files with matches,
suitable for piping to xargs. Both mmap and no-mmap searches terminate
after the first match is found. Documentation updated and tests added.

ci/script.sh

@@ -16,6 +16,7 @@ disable_cross_doctests() {
 }
 
 run_test_suite() {
+    cargo clean --target $TARGET --verbose
     cargo build --target $TARGET --verbose
     cargo test --target $TARGET --verbose