Search in current directory or marked files

Hi!
I made  a little command for search in current folder or marked files.
These six lines could be
```python
where = ', '.join(self.get_marked()) or self.path or ''
```

Also, I think it should be called ‘Find in files’, because that’s what it is; and ‘Search’ is ambiguous, may imply, e.g. search filenames.
Thank you for pull request.