Download existing release assets before uploading new ones

[AdmiringWorm]

When the command add assets is executed, we may wish to download the existing release assets to verify that they are still the same as the ones specified in the sha (when enabled), or rather to update the sha checksums.
This can make it easier in the future as well to extend the templating to allow the whole sha section to be specified in a template file instead.

Opening this up mainly for discussion first.

/cc @gep13 @akordowski