feat: add alt-gutter color option (#4602)

* Add alt-gutter color option

* Simplify the code

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Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

man/man1/fzf.1

@@ -272,6 +272,7 @@ color mappings. Each entry is separated by a comma and/or whitespaces.
       \fBgutter              \fRGutter on the left
     \fBcurrent\-hl (hl+)      \fRHighlighted substrings (current line)
     \fBalt\-bg                \fRAlternate background color to create striped lines
+    \fBalt\-gutter            \fRAlternate gutter color to create the striped pattern
     \fBquery (input\-fg)      \fRQuery string
       \fBghost               \fRGhost text (\fB\-\-ghost\fR, \fBdim\fR applied by default)
       \fBdisabled            \fRQuery string when search is disabled (\fB\-\-disabled\fR)