Topic seek mods

### In `topic_seek.py`
* Cosmetic changes
* Made `paths` and `doc_ids` both `@lazyprop` decorated
* Set `self.formatter` based on `__init__` arguments...this is used in place of hard-coded `VWFormatter`
* Use `dictionary.filter_extremes` built in method, rather than custom method
* Made sure `limit` kwarg was used everywhere.
* Removed `num_topics` kwarg from `write_topics()`
* Removed setting of `self.corpus` and `self.dictionary` to `None` before actual initialization....not really necessary...but it's hard to keep track of what has been set where and then raise the appropriate warning---so might as well just let an exception be raised and then look at the traceback....the traceback and custom warning were basically the same.

### In `text_processors.py`
* Added `limit` kwarg to `sfile_to_token_iter`
* Using `doc_id` rather than `tag`
* Using `sstr` all around now

### In `files_to_vw.py`
* Kept up with changes to `text_processors.py`
* Using an iter now for base paths