Create a dataset loader for MedQuAD

## Adding a Dataset
- **Name:** *MedQuAD*
- **Description:** *MedQuAD includes 47,457 medical question-answer pairs created from 12 NIH websites (e.g. cancer.gov, niddk.nih.gov, GARD, MedlinePlus Health Topics). The collection covers 37 question types (e.g. Treatment, Diagnosis, Side Effects) associated with diseases, drugs and other medical entities such as tests.*
- **Task:** *QA, RQE*
- **Paper:** *https://bmcbioinformatics.biomedcentral.com/articles/10.1186/s12859-019-3119-4*
- **Data:** *https://github.com/abachaa/MedQuAD*
- **License:** *CC BY 4.0*
- **Motivation:** *Study biomedical Q&A with answers from reliable sources*