docslab mdBook preprocessor

This mdBook preprocessor uses docslab to make code blocks interactive.

First, install the mdbook-docslab crate:

cargo install mdbook-docslab

Then, configure your book source to use it,

mdbook-docslab install path/to/your/book

which will not modify existing values if the book is already using docslab. In other words, it is safe to run mdbook-docslab install more than once.

To make a code block interactive, add docslab to the info string, e.g.,

```cpp docslab
// hardshare: heliumdev/cubecell-draw-demo
// command: source .platformio/penv/bin/activate && cd m/Heltec-CubeCell-Board/examples/cubecell-helium-us915-basic && pio run -t upload && pio device monitor
// destpath: m/Heltec-CubeCell-Board/examples/cubecell-helium-us915-basic/src/main.cpp
// repoUrl: https://github.com/helium/longfi-platformio.git
// ---
#include "Arduino.h"

void setup()
{
    Serial.begin(115200);
}

void loop()
{
    delay(1000);
    Serial.println("Hola, mundo!");
}
```