fix: Use '/' path separator on Windows for tar archives (#24)

Creating a tar archive on Windows with a directory results instead in the file having '\\' in its name,
and on extraction on POSIX, does not create the appropriate directory.

To fix this, use `path.posix` to force the '/' path separator even if creating the archive on Windows.

closes #22

lib/tar/stream.js

@@ -75,12 +75,12 @@ class TarStream extends BaseStream {
       files.forEach(fileOrDir => {
         const newOpts = utils.clone(opts);
         if (opts.ignoreBase) {
-          newOpts.relativePath = path.join(relativePath, fileOrDir);
+          newOpts.relativePath = path.posix.join(relativePath, fileOrDir);
         } else {
-          newOpts.relativePath = path.join(relativePath, path.basename(entry), fileOrDir);
+          newOpts.relativePath = path.posix.join(relativePath, path.basename(entry), fileOrDir);
         }
         newOpts.ignoreBase = true;
-        this.addEntry(path.join(entry, fileOrDir), newOpts);
+        this.addEntry(path.posix.join(entry, fileOrDir), newOpts);
       });
       this._onEntryFinish();
     });