Add a parameter in `add_flat_template()` to specify the name of the vignette

[MargotBr]

The function add_flat_template() has a flat_name to choose the name of the Rmd file.

But there is no possibly to choose the name of the vignette.

It will be nice to be able to run:

fusen::add_flat_template(template = "additional",
                                            flat_name = "check_data", 
                                            vignette_name = "Check data")

And to have the corresponding inflate chunk at the end of the created file flat_ check_data.Rmd:

fusen::inflate(flat_file = "dev/flat_check_data.Rmd", 
                       vignette_name = "Check data")