Add queryable QTL types

[bschilder]

• exon expression QTL: quant_method="ex"
• transcript usage QTL: quant_method="tx"
• promoter, splice junction and 3' end usage QTL: quant_method="txrev"

[bschilder]

The eQTL Catalogue API supports these QTL types (exon, transcript, txrev) and they can be queried by passing the appropriate quant_method to the API. The current catalogueR functions pass through API parameters, so users can already filter by quant_method in their queries. Closing as the functionality is accessible, though not explicitly parameterized in the high-level wrapper.