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

man/man1/fzf.1

@@ -270,6 +270,7 @@ color mappings.
     \fBcurrent\-bg (bg+)      \fRBackground (current line)
       \fBgutter              \fRGutter on the left
     \fBcurrent\-hl (hl+)      \fRHighlighted substrings (current line)
+    \fBalt\-bg                \fRAlternate background color to create striped lines
     \fBquery (input\-fg)      \fRQuery string
       \fBdisabled            \fRQuery string when search is disabled (\fB\-\-disabled\fR)
     \fBinfo                  \fRInfo line (match counters)