custom yaml header fields in the Download Modules

[Polkas]

The developer should have an option to specify which yaml header fields should be ask to be filled by the end user.
We are talking about fields like author, output, etc.

shiny inputs connected with yaml header fields should be named the same, input$author for author field ... .
Should be possible after #11 as there will be a function to process the flat list of yaml header fields.

We will add an argument to Download module with a vector of yaml files which should be taken into account.

UI for Download Button and Previewer have to be updated.