add: rewriter logic and tests

src/testdata/testdst/hello.txt

@@ -0,0 +1,5 @@
+This is a line
+This is abc line
+This is def line
+
+This is another line

src/testdata/testdst/recursive/world.txt

@@ -0,0 +1,4 @@
+This is a abc line.
+This is a def line.
+
+In a nested directory.

src/testdata/testsrc/hello.txt

@@ -0,0 +1,4 @@
+This is a line
+This is to_be_replaced line
+
+This is another line

src/testdata/testsrc/recursive/world.txt

@@ -0,0 +1,3 @@
+This is a to_be_replaced line.
+
+In a nested directory.