Feature: Subfolders

**Feature:** Support subfolders / categories structure.

**Mockup:**
```
http://localhost:7071/api/promos
http://localhost:7071/api/promos/list
http://localhost:7071/api/promos/newest
```
```
promos/
├── function.json
├── index.js
├── list
│   ├── function.json
│   └── index.js
└── newest
├── function.json
└── index.js
```

---

Existing workarounds are rather janky:
- Create custom proxy paths mapping
- Manual, One by one, and hardcoded
- Folder structure is still all in root (since `function.json` can't be moved)
- "Standard Functions billing applies to proxy executions." ([ref](https://docs.microsoft.com/en-us/azure/azure-functions/functions-proxies#create))
- Use [Configure Script entry point](https://docs.microsoft.com/en-us/azure/azure-functions/functions-reference-node#configure-function-entry-point)
- Manual, One by one, and hardcoded
- Folder structure is still all in root (since `function.json` can't be moved)

Stackoverflow request: https://stackoverflow.com/questions/44176892/azure-functions-in-sub-folders