support update using URLs instead of namespaces

[jhpoelen]

Currently, namespace (e.g., globalbioticinteractions/ucsb-izc) are used to update individual datasets using:

$ elton update globalbioticinteractions/ucsb-izc

where the namespace is available from list provided via elton ls after a full update elton update.

However, in the background, these namespaces are translated to github repositories (or their Zenodo counterparts) (e.g., https://github.com/globalbioticinteractions/ucsb-icz).

Suggest to directly support:

$ elton update https://github.com/globalbioticinteractions/ucsb-icz

fyi @seltmann

[jhpoelen]

related to #47