Improve include

[rikhuijzer]

When inserting dynamic content via

```{.include}
_gen/foo.md
```

the output will always have spacing around it.
This is inconvenient when you would like to write something like "This project is built with Julia $(version)" or to report a statistic like "the mean is $(mean_x).

So, this can be improved in two ways:

1. Allow include without spacing around it. This would look bad in the markdown, but good in the end-result
2. Allow in-line includes like presented above $(version).

[rikhuijzer]

It's probably time to create a proper lua filter which converts statements like foo (even inline) to _gen/foo.md.

Also, it should support arguments, so that var("a") would give as output 1 when a user defines:

function var(s::String)
    d = Dict(
        "a" => 1
        "b" => 2
    )
    d[s]
end

This could even support accessing objects inside the module, because the text is just parsed. I should ensure that " is converted to something like %22 in the filename.

[rikhuijzer]

If functions can be called, this basically yields the same possibilities as Bookdown, because

```gen
cb(function_name; output=false)
```

is similar to

```{r, echo=FALSE}
function_name()
```

[rikhuijzer]

It appears that

`jl foo()` 

can only contain inline elements, so no full Markdown document can be parsed. That's okay, I guess. It will just put text there.