!220 add 增加年月格式化常量

Merge pull request !220 from jiangzeyin/v5-dev

hutool-core/src/main/java/cn/hutool/core/date/DatePattern.java

@@ -29,6 +29,30 @@ public class DatePattern {
 
 	//-------------------------------------------------------------------------------------------------------------------------------- Normal
 	/**
+	 * 年月格式：yyyy-MM
+	 */
+	public static final String NORM_MONTH_PATTERN = "yyyy-MM";
+	/**
+	 * 年月格式 {@link FastDateFormat}：yyyy-MM
+	 */
+	public static final FastDateFormat NORM_MONTH_FORMAT = FastDateFormat.getInstance(NORM_MONTH_PATTERN);
+	/**
+	 * 年月格式 {@link FastDateFormat}：yyyy-MM
+	 */
+	public static final DateTimeFormatter NORM_MONTH_FORMATTER = DateTimeFormatter.ofPattern(NORM_MONTH_PATTERN);
+	/**
+	 * 简单年月格式：yyyyMM
+	 */
+	public static final String SIMPLE_MONTH_PATTERN = "yyyyMM";
+	/**
+	 * 简单年月格式 {@link FastDateFormat}：yyyyMM
+	 */
+	public static final FastDateFormat SIMPLE_MONTH_FORMAT = FastDateFormat.getInstance(SIMPLE_MONTH_PATTERN);
+	/**
+	 * 简单年月格式 {@link FastDateFormat}：yyyyMM
+	 */
+	public static final DateTimeFormatter SIMPLE_MONTH_FORMATTER = DateTimeFormatter.ofPattern(SIMPLE_MONTH_PATTERN);
+	/**
 	 * 标准日期格式：yyyy-MM-dd
 	 */
 	public static final String NORM_DATE_PATTERN = "yyyy-MM-dd";