Add selection color to the theme config

[tranzystorekk]

Looking at a typical terminal color template like: https://github.com/mbadolato/iTerm2-Color-Schemes/blob/master/README.md#how-to-add-new-template

rio is missing at least a selection field in the [colors] config,

To consider if cursor text color and selection text color are desired as well.

[raphamorim]

For this issue would recommend 3 new properties for theme configuration:

selection-foreground = "000000"
selection-background = "000000"
selection-cursor = "000000"

[tranzystorekk]

selection-cursor seems unnecessary - typically a user probably won't care enough

[raphamorim]

Yeah but is no brainer to implement, so rather keep as personalisation option (it can just inherits the cursor color if is not set)

[raphamorim]

Hmm, to avoid complexity ignored the selection-cursor as you suggested.

Implemented in 9f64b1a