v0.4.0

new feature

• save/load descriptors list to json. (#17, 13e1bf7)
• add is_missing function (f397800)
• web UI (mordred-web packge: https://github.com/mordred-descriptor/mordred-web)

broken change

• raise TypeError when Calculator/Descriptor is provided non-Mol values. (thanks @RunshengSong #16, #20, d584cb3)