Add command line flag to limit tasks

[sjthespian]

I have a dregsy yaml file that syncs to 5 different docker registries. From time to time, I would like to be able to run a manual sync to just one or two of them w/o having to extract that data to a separate yaml file. It would be very useful to be able to specify which tasks to run from the command line. I'm thinking something along the lines of what anisble-playbook uses:
dregsy --config regsync.yml --limit sync-registry2

[xelalexv]

Interesting. I see how that could be handy. We could go for two complementary flags though, --select and --deselect, or alternatively --include/--exclude. Not sure yet which one is better.

In the meantime, you could use yq to filter your config file, e.g.:

# include all test-* tasks
yq eval 'del(.tasks.[] | select(.name != "test-*"))' dregsy.yaml

# exclude all test-* tasks
yq eval 'del(.tasks.[] | select(.name == "test-*"))' dregsy.yaml

There should be an expression with only select, but I couldn't figure out how to do that just now.

[rubin55]

What about --task which if not specified simply does all tasks defined in the configuration file, but can be specified multiple times to select tasks to run, like:

$ dregsy --config my_config_with_many_tasks.yaml --task foo --task bar --task baz

[xelalexv]

I finally settled on -run {regexp}, similar to go test -run {regexp}. This selects the tasks to be run. Only difference to go test is that here the expression already does a line match, so you don't need to place the expression in ^...$ to get an exact match. For example, -run=task-a will only select task-a, but not task-abc.

[xelalexv]

done with PR #76