[feature] let default plugin is current directory

[Freed-Wu]

❯ vimdoc
usage: vimdoc [-h] [--version] PLUGIN
vimdoc: error: the following arguments are required: PLUGIN
❯ vimdoc .

Why not let default plugin is .? Thanks.

[dbarnett]

I kinda prefer having it explicit over the small convenience of not having to type ..

Might be worth giving a better error with a usage hint when plugin path is missing, though.

[Freed-Wu]

OK, I see.

[dbarnett]

Turned out argparse didn't make it as easy as I'd expected to add a custom error for that case like "Did you mean to run 'vimdoc .'?" But I have some changes for slightly more self-explanatory help on the PLUGIN arg in #126.