feat(step): add new modify step

lib/step/modify.js

@@ -0,0 +1,33 @@
+const replace = require('stream-replace');
+
+module.exports = ({modify} = null, stream) => {
+  if (!modify) {
+    return stream;
+  }
+
+  const {patch} = modify;
+
+  patch.forEach(({pattern, append, prepend, custom}) => {
+    stream = stream.pipe(applyModification(pattern, {append, prepend, custom}));
+  });
+
+  return stream;
+};
+
+const applyModification = (pattern, {append, prepend, custom}) => {
+  return replace(pattern, (match, parameter, defaultValue) => {
+    if (typeof append !== 'undefined') {
+      return match + append;
+    }
+
+    if (typeof prepend !== 'undefined') {
+      return prepend + match;
+    }
+
+    if (typeof custom === 'function') {
+      return custom(match, parameter, defaultValue);
+    }
+
+    return match;
+  });
+};