minor UI issue: "parser" specific options are not listed in --help

e.g.
```
$> tributors init zenodo
Please provide the zenodo doi with --doi
```
but
```
$> tributors init zenodo --help
usage: tributors init [-h] [--force] [--repo REPO]
[{zenodo,allcontrib,codemeta,all} [{zenodo,allcontrib,codemeta,all} ...]]

positional arguments:
{zenodo,allcontrib,codemeta,all}
Metadata file parsers to update or initialize.

optional arguments:
-h, --help show this help message and exit
--force If exists, overwrite existing .all-contributorsrc
--repo REPO The repository URI, if not exported to GITHUB_REPOSITORY
```
I guess `--help` could list options for all the parsers and annotate to which any applies?