add number validator for --authors flag

Author: o2sh

Diff for: resources/go.ascii

@@ -3,7 +3,7 @@
 {0} .==  =o={1}oGGGGGG{0}o=oo=o{1}GGGGGGG{0}G=o=  oo-
 {0} -o= oo={1}G .=GGGGG{0}o=o={1}= .=GGGGG{0}=ooo o=-
 {0}  .-=oo={1}o==oGGGGG{0}=oo={1}oooGGGGGo{0}=oooo.
-{0}   -ooooo={1}oooooo{0}={2}.   .{0}={1}=ooo=={0}oooooo-
+{0}   -ooooo{1}=oooooo{0}={2}.   .{0}={1}=ooo=={0}oooooo-
 {0}   -ooooooooooo{2}====_===={0}ooooooooooo=
 {0}   -oooooooooooo{2}=={1}#{0}.{1}#{2}=={0}ooooooooooooo
 {0}   -ooooooooooooo={1}#{0}.{1}#{0}=oooooooooooooo

Diff for: src/onefetch/cli.rs

@@ -133,7 +133,15 @@ impl Cli {
                 .takes_value(true)
                 .max_values(1)
                 .default_value("3")
-                .help("NUM of authors to be shown."),
+                .help("NUM of authors to be shown.")
+                .validator(
+                    |t| {
+                        t.parse::<u32>()
+                            .map_err(|_t| "must be a number")
+                            .map(|_t| ()) 
+                            .map_err(|e| e.to_string())
+                    },
+                )
         )
         .arg(
             Arg::with_name("exclude")