Adds automatic shifting

[ghost]

Enables auto-shifting titles With this feature all your files can begin with a level 1 heading. When included, it will shift accordingly to the previous title found.

Also adds a warning message if file is not found rather than crash

[tarleb]

Sorry for letting this lay around for so long, and thank you for your patience.

This looks nice, and I'd like to merge it. Maybe we could add a little warning that the result may not be as expected in certain cases. E.g., some people might expect the include to be inserted at level 1, but it would be inserted at level 2 here:

# Title

:::{.wrapping-div}
## level 2
:::

```{.include}
file.md

Could you break the code changes up into two parts?  The warning message is a good idea and should be in its own commit, or even PR. The rational here is that, if we ever have to revert a feature for whatever reason, it is much easier to do so if only a single relevant commit has to be reverted.

Finally a request about style: the new code mixes camelCase with snake_case; we try to keep things consistent by following the [Olivine labs style guide](https://github.com/Olivine-Labs/lua-style-guide). The suggested naming style for variables is snake_case.

Cheers!

[ghost]

Thanks for your advice. Here my to do list :

• [x] Add a https://github.com/pandoc/lua-filters/pull/117 with the warning for not found file
• [x] Review styling on this PR
• [x] Add a better explanation in the readme on what is the default level of injection and how to specify the desired one
• [x] Fix some typos

[ghost]

@tarleb MR is ready. Review it when you can, thx