Add 'alt-bg' color for striped lines (#4370)

Test cases:

1. 'jump' should show alternating background colors even when 'alt-bg' is
not defined as before.

  go run main.go --bind load:jump

Two differences:
  * The alternating lines will not be in bold (was a bug)
  * The marker column will not be rendered with alternating background color

2. Use alternating background color when 'alt-bg' is set

  go run main.go --color bg:238,alt-bg:237
  go run main.go --color bg:238,alt-bg:237 --highlight-line

3. 'selected-bg' should take precedence

  go run main.go --color bg:238,alt-bg:237,selected-bg:232 \
                 --highlight-line --multi --bind 'load:select+up+select+up'

4. Should work with text with ANSI colors

  declare -f | perl -0777 -pe 's/^}\n/}\0/gm' |
    bat --plain --language bash --color always |
    go run main.go --read0 --ansi --reverse --multi \
                   --color bg:237,alt-bg:238,current-bg:236 --highlight-line

---

Close #4354
Fix #4372

src/options.go

@@ -1295,6 +1295,8 @@ func parseTheme(defaultTheme *tui.ColorTheme, str string) (*tui.ColorTheme, erro
 				mergeAttr(&theme.Current)
 			case "current-bg", "bg+":
 				mergeAttr(&theme.DarkBg)
+			case "alt-bg":
+				mergeAttr(&theme.AltBg)
 			case "selected-fg":
 				mergeAttr(&theme.SelectedFg)
 			case "selected-bg":