Fix fields argument to CLI

[jsignell]

Related Issue(s):

• Closes Unable to define multiple exclusions on --fields option #796

Description:

This is not the most beautiful PR, but this now works:

stac-client search https://earth-search.aws.element84.com/v1 -c sentinel-2-l2a --bbox -72.5 40.5 -72 41 --datetime 2020-01-01/2020-01-11 --fields=-geometry,-assets --max-items 1

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to CHANGELOG.md