Instead of assuming we're always compiling from `slides.rst`, make the
filename dynamic so it can build whatever files are needed.
For example, to generate the demo slides for Test Driven Drupal, I can
run:
```shell
just generate-pdf ./src/test-driven-drupal demo.rst
```
If no filename is specified, default to `slides.rst` and all the slides
will be compiled.
Pass the path to the talk directory - e.g.
`just generate ./src/test-driven-drupal` instead of just the talk
name/slug.
This doesn't change where the files are sourced from or output, but this
makes it easier to use `just generate` as I can use tab completion when
executing the command - making it quicker and less prone to typos.
Oliver Davies