Add unixtime_millis, unixtime_micros and unixtime_nanos

This commit adds the options introduced by
fluent/fluentd#3220.

Signed-off-by: abicky <[email protected]>

Author: abicky

configuration/inject-section.md

@@ -67,12 +67,24 @@ Injected Record:
   type
 
   * Default: `float`
-  * Available values: `float`, `unixtime`, `string`
-    * `float`: seconds from Epoch + nano seconds \(e.g.
+  * Available values:
+    * `float`: seconds from Epoch + microseconds \(e.g.
 
-      1510544836.154709804\)
+      1510544836.154709\)
+
+    * `unixtime`: seconds from Epoch \(e.g. 1510544836\)
+    * `unixtime_millis`: milliseconds from Epoch \(e.g.
+
+      1510544836154\)
+
+    * `unixtime_micros`: microseconds from Epoch \(e.g.
+
+      1510544836154709\)
+
+    * `unixtime_nanos`: nanoseconds from Epoch \(e.g.
+
+      1510544836154709804\)
 
-    * `unixtime`: seconds from Epoch \(e.g. 1510544815\)
     * `string`: use format specified by `time_format`, local time or time
 
       zone