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

[yarikoptic]

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

• Does not expand ~ #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???