function resolveFilePath(filePath, extension) {

  var resolvedPath = di.path.resolve(filePath);

  // The extension is not added
  if (di.path.extname(resolvedPath) != '.' + extension) {
    resolvedPath += '.' + extension;
  }

  return resolvedPath;

}
Resolve to an absolute path

Accepts
  - FileName
  - FileName.ext
  - /path/to/FileName
  - /path/to/FileName.ext

- Add the extension if not already added
- Resolve to `/path/to/FileName.ext`

@param  {String} filePath  an absolute or a relative path
@param  {String} extension
@return {String}