allow rendering included files

[flokli]

I have an external process creating a .gv file, and I want to use the {{#include path/to.gv}} directive to include the file:

{{#include path/to.gv}}

This fails, as apparently the include directive is interpreted as raw graphviz code:

2022-04-27 11:34:10 [INFO] (mdbook::book): Book building has started
Warning: The graphviz plugin was built against version 0.4.15 of mdbook, but we're being called from version 0.4.17
Error: <stdin>: syntax error in line 1 near '{'
Error response from Graphviz
2022-04-27 11:34:10 [ERROR] (mdbook::utils): Error: The "graphviz" preprocessor exited unsuccessfully with exit status: 1 status

It would be nice if this was supported, or another way to embed a rendered .gv into mdbook.

[dylanowen]

I haven't played around with included files yet but was this with something like:

```dot process
{{#include path/to.gv}}
```

?

[dylanowen]

Also maybe https://rust-lang.github.io/mdBook/format/configuration/preprocessors.html?highlight=preprocessors#require-a-certain-order could help

[flokli]

I haven't played around with included files yet but was this with something like:

```dot process
{{#include path/to.gv}}

?

Yeah, sorry, that's what I wanted to write, but github did interpret it.

[dylanowen]

Gotcha, yeah you have to layer the code block deliminators https://github.com/dylanowen/mdbook-graphviz/blob/master/Readme.md?plain=1#L34

[flokli]

@dylanowen yes, you were right - using the preprocessors and their orders was the right fix. I opened #33, which adds a section into the Readme explaining this. PTAL!