Revise and add arguments to `theme_unhcr` function

[vidonne]

The arguments of the theme_unhcr function provide sensible default options to create UNHCR branded plots but also give the user the flexibility to change those default values but still follow UNHCR data visualization recommendations.

The idea of this enhancement is to change some current arguments to more adequate default values and add some argument to provide more flexibility and easier alteration of the plot.

1. Current argument to change:

• font_size from 9 to 11 (or 12)
• grid from TRUE to "XY"
• axis from "x" to FALSE, to avoid the expand issue

2. Current argument to repurpose: Keep the following arguments for back compatibility but won't be central to the call anymore.

• line_size
• rel_small
• rel_tiny
• rel_large

3. New arguments to add:

• plot_title_size: Title font size
• plot_title_margin: Margin below title
• subtitle_size: Subtitle font size
• subtitle_margin: Margin below subtitle
• strip_text_size: facet title font size
• caption_size: Caption font size
• caption_margin: Margin above the caption
• axis_text_size: Axis label font size
• axis_title_size: Axis title font size
• legend_text_size: Legend keys font size
• legend_title_size: Legend title font size
• plot_margin: Overall margins around the plot
• void: Remove all plot decoration, useful for map and pie