Add guard/warning for `~` in file path or do expand

I got burnt again by it while doing

```
(git)smaug:~/proj/openneuro/OpenNeuroStudies[master]git
$> datalad run -m "Re-extracting metadata about the state after fixups" duct -p '~/.cache/duct/logs/ons-{{datetime_filesafe}}-{{pid}}_' make metadata CORES=4
```

and then not finding logs under `~/.cache`. Prior/original cases like that were in

- #94

and I agreed that it should be shell's job. Here though i quoted the path which forbid shell from expanding. Again -- kinda a user error. But since I think it would be atypical to desire `~/` folder -- I think we should expand it if it leads the path, or at least warn about it???

Start by tracing how the duct CLI handles the -p path argument in the example command, then run the quoted ~/ path case to observe where it is stored. Done should be a decided and tested behavior for a leading ~: either expand it or emit a warning, while preserving ordinary quoted paths.

Written by the indexing model from the issue text.