Filter to include code from source files.

The filter is largely inspired by pandoc-include-code.

The filter recognizes code blocks with the include attribute present. It swaps the content of the code block with contents from a file.

The simplest way to use this filter is to include an entire file:

```{include="hello.c"}
```

You can still use other attributes, and classes, to control the code blocks:

```{.c include="hello.c" numberLines}
```

If you want to include a specific range of lines, use startLine and endLine:

```{include="hello.c" startLine=35 endLine=80}
```

start-line and end-line alternatives are also recognized.

Using the dedent attribute, you can have whitespaces removed on each line, where possible (non-whitespace character will not be removed even if they occur in the dedent area).

```{include="hello.c" dedent=4}
```

If you include the numberLines class in your code block, and use include, the startFrom attribute will be added with respect to the included code's location in the source file.

```{include="hello.c" startLine=35 endLine=80 .numberLines}
```

An HTML can be produced with this command:

pandoc --lua-filter=include-code-files.lua sample.md --output result.html