add script to download and update holiday CSV file

.gitignore

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+*.swp
+*.csv

clean

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+#!/bin/sh
+rm -f *.csv

update

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+#!/bin/sh
+
+# Settings
+CSV_URL="https://www8.cao.go.jp/chosei/shukujitsu/syukujitsu.csv"
+
+# Internal configuration settings
+TMP_CSV_FILE="holidays-jp-tmp.csv"
+RAW_CSV_FILE="holidays-jp-raw.csv"
+CSV_FILE="holidays-jp.csv"
+
+EXIT_SUCCESS=0
+EXIT_FAILURE=1
+
+# Verify the required runtime environment
+if ! command -v curl > /dev/null; then
+  echo "This script requires \"curl\" to be installed. Please install it and try again."
+  exit $EXIT_FAILURE
+fi
+if ! command -v nkf > /dev/null; then
+  echo "This script requires \"nkf\" to be installed. Please install it and try again."
+  exit $EXIT_FAILURE
+fi
+
+# Run
+curl -sS -L -o $TMP_CSV_FILE $CSV_URL
+if ! diff -q $RAW_CSV_FILE $TMP_CSV_FILE > /dev/null 2>&1; then
+  mv $TMP_CSV_FILE $RAW_CSV_FILE
+  nkf -w $RAW_CSV_FILE > $CSV_FILE
+else
+  rm $TMP_CSV_FILE
+fi
+
+# Exit
+exit $EXIT_SUCCESS