[Feature Request] Allow multiple roots

Nobody has claimed this yet.

Consider the following project tree:
```
project-root
├ src
│ └ main.js
└ static
└ index.html
```
Currently, for `index.html` be able to link to `main.js`, servor must run in the `project-root` folder and the script tag in html must be ``
```shell
$ cd ~/project-root
$ servor .
```

It would be really useful to allow servor to serve more folders than just the root. Something like:
```shell
$ cd ~/project-root
$ servor ./static --extern ./src
```
This would serve the `./src` folder as if it and `./static` were the same.
So the `index.html` would link the script at same level (`<script src="./main.js">`)

The `--extern` param could be repeatable to allow multiple entrypoints
```shell
$ cd ~/project-root
$ servor ./static --extern ./src --extern ./libs
```
Duplicate paths (e.g.: `./static/style.css`, `./src/style.css` and `./libs/style.css`) could be served in order (left-to-right), returning the first found path.

Start by tracing the `servor ./static` CLI entry point and how requested paths are resolved and served. Check how repeated `--extern` arguments could be parsed and how duplicate paths should be searched left-to-right. Done means static and external folders share URL paths, with the first matching file returned.

Written by the indexing model from the issue text.