[Feature Request] Allow multiple roots

[NemoStein]

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 <script src="../src/main.js">

$ cd ~/project-root
$ servor .

It would be really useful to allow servor to serve more folders than just the root. Something like:

$ 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

$ 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.