feat: add a path option

[nikpivkin]

Closes #73

[sagikazarmark]

I wonder if it would make sense to make the path a positional argument instead for the relevant commands.

licensei check path/to/dir
licensei cache path/to/dir
licensei list path/to/dir

I could even imagine adding support for multiple paths in the future.

What do you think?

[jaxxstorm]

I think both are valid, positional argument probably has the best UX

[nikpivkin]

@sagikazarmark Sounds reasonable, I'll fix it.

[skyzyx]

I would love to not have to set a path at all. It would be simpler (for users) to be able to scan all child go.mod files automatically.