feat: allow to specify subdirectory as source

[norskeld]

While you can't download a specific directory from a repository using GitHub API or direct links, such functionality would be useful/desirable in some cases.

For instance, we may have a repository with multiple templates located in different subdirectories:

repository
├── code-style
│   └── decaff.kdl
├── frontend
│   └── decaff.kdl
└── server
    └── decaff.kdl

We should be able to use one of them as the template. What should be added:

• CLI option, e.g. decaff remote user/repo#ac75a88 --path frontend
• Ability to specify a directory using the source syntax, e.g. decaff remote user/repo@frontend#ac75a88