Feature request: Add margins parameter

[Britman72]

Not every screenplay software, nor writer for that matter, uses the same page formatting. So it would be great to have parameters to override the margins when rendering to PDF. As an example, I am converting from FDX to Fountain and then to PDF. Compared to the original FDX, the margins are slightly off which results in an extra 6 pages.

[lichendust]

This will be added in some form in v0.3.0.

One of the next steps is to externalise all templating so everyone can fully customise their output.