Support plot_title_position/plot_caption_position parameters

[ASmirnov-HORIS]

In ggplot2 there is parameters plot.title.position and plot.caption.position which can take two string values:

• "panel" (default) - titles and/or caption are aligned to the plot panels:

• "plot" - titles and/or caption are aligned to the entire plot:

  Presumably this is the only valid way to move the titles outside the panel.

[alshan]

See blogpost: https://www.ddanieltan.com/posts/30-day-chart-1/

[OLarionova-HORIS]

Added: https://github.com/JetBrains/lets-plot/commit/36f33f3ee1112e35d4677d6f3f313e37ab48f878