Add monomer library functionality 🧪🧪🧪

This release adds two new functions:

1. chembl_downloader.download_monomer_library which gets this file https://ftp.ebi.ac.uk/pub/databases/chembl/ChEMBLdb/latest/chembl_30_monomer_library.xml for whatever version you specify
2. chembl_downloader.get_monomer_library_root which does the same as the downloader but also parses the XML for you

Thanks to @iwatobipen and his recent blog post for inspiring this.