WIP: New corpora

### Added new "gensim corpus compatible" classes in `gensim_helpers`. 
* `SvmLightPlusCorpus` is a subclass of `gensim.corpora.SvmLightCorpus` that gives acces to, and methods to filter with, `doc_id` 
* `StreamerCorpus` can stream tokens from text files and sort of works like a corpus

### Modifications to `TextFileStreamer`
* Small differences to use of `tokenizer` and `limit`
* Compatibility with `Topic` class not checked

### `filter_with_meta` notebook
* Shows use of new corpus/streamer