Expose options.base for dest

Because the stream returned from `dest` is also readable, it is kinda link a `src` to the next `dest` so the `base` should be configurable just like `src`.

README.md

@@ -90,6 +90,8 @@ This is just [glob-watcher].
 - Possible options for the second argument:
   - cwd - Specify the working directory the folder is relative to.
     - Default is `process.cwd()`.
+  - base - Specify the folder relative to the cwd. This is used to determine the file names when saving in `.dest()`.
+    - Default is the `cwd` resolves to the folder path.
   - mode - Specify the mode the files should be created with.
     - Default is the mode of the input file (file.stat.mode) if any.
     - Default is the process mode if the input file has no mode property.
@@ -112,6 +114,8 @@ This is just [glob-watcher].
 - Possible options for the second argument:
   - cwd - Specify the working directory the folder is relative to.
     - Default is `process.cwd()`.
+  - base - Specify the folder relative to the cwd. This is used to determine the file names when saving in `.dest()`.
+    - Default is the `cwd` resolves to the folder path.
   - dirMode - Specify the mode the directory should be created with.
     - Default is the process mode.
 - Returns a Readable/Writable stream.

lib/prepareWrite.js

@@ -23,7 +23,7 @@ function prepareWrite(outFolder, file, opt, cb) {
   var outFolderPath = (
     typeof outFolder === 'string' ? outFolder : outFolder(file)
   );
-  var basePath = path.resolve(cwd, outFolderPath);
+  var basePath = options.base || path.resolve(cwd, outFolderPath);
   var writePath = path.resolve(basePath, file.relative);
   var writeFolder = path.dirname(writePath);