Template library config option

[Keats]

Closes #399

[Keats]

The 2 placeholders:

• r_version: expands to {major}.{minor}
• name: project.name without any validation

With config library = "libs/{r_version}/{name}":

example_projects/custom-lib-path/libs
└── 4.5
    └── simple
        ├── R6
        │   ├── Meta
        │   ├── R
        │   ├── help
        │   │   └── figures
        │   └── html
        ├── cli