Feat: tsrc add to add a new repository to the manifest

**Is your feature request related to a problem? Please describe.**
It is hard to add a new repository to a manifest.

**Describe the solution you'd like**

```bash
$ tsrc add <local-path>
would you like to add <local-path> to your manifest:
path: <local-path>
url: <git-url>
branch: <branch>
```

Yeah something along those lines :)
