Support having multiple versions of containers, and specification of version upon execution

[yarikoptic]

Idea was formulated by @jbwexler in

• Add ability to specify version of container when running ReproNim/containers#114
  and does have good meaning there given that the dataset has a good number of versions for many containers. Overall it could boil down to

1. specification @jbwexler suggested there to have smth like

[datalad "containers.bids-fmriprep]
    cmdexec = {img_dspath}/scripts/singularity_cmd run {img} {cmd}
    image = images/bids/bids-fmriprep--{version}.sing
    default_version = 20.2.0

or even just

[datalad "containers.bids-fmriprep]
    cmdexec = {img_dspath}/scripts/singularity_cmd run {img} {cmd}
    image = images/bids/bids-fmriprep--{version}.sing
    version = 20.2.0

hence version specifying the default one but us allowing to overload it while naming which container to run:

2. making it possible to specify version while running e.g. as -n {NAME}={version} or adding explicit --version to overload that version in the specification.