Ability to specify additional tags via from-args subcommand

[neanton]

Only from-spec subcommand allowed specifying extra tags for manifest list, but this is not convinient to create or modify manifest YAML during CI. This PR adds additional --tags argument to from-args subcommand to specify tags via command-line without touching/creating/modifying manifest during CI.

Also this PR includes minor refactoring to required cli args and adds an example to README.md

[estesp]

Can you re-format your commit message as

short descriptive title

additional detail

Signed-off-by: ...

The DCO check also checks for this formatting; yours can be easily fixed with:

ability to specify additional tags via from-args 

from-args subcommand now allows extra tags to be specified
also mark arguments as required, additions to readme

Signed-off-by: Neznaenko Anton <neanton@gmail.com>

[neanton]

Done, thanks!