Create dataset loader for Plant-Disease corpus

## Adding a Dataset
- **Name:** *plant-disease*
- **Description:** *Dataset with tagged plant/disease entities, as well as relations on how the plants affect the tagged diseases*
- **Task:** *NER, RE*
- **Paper:** [*A corpus of plant–disease relations in the biomedical domain*
](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0221582)- **Data:** http://gcancer.org/pdr/
- **License:** *None*
- **Motivation:** *This is a robust benchmark for NER and RE models, as well as a useful resource for NLP-based identification of plants that can be used to treat diseases.*