Add feature toggle to read numeric strings as numeric timestamps

There is an apparent inconsistency in the way Jackson de-serializes numbers
that are shaped as a string into an instant.

 * When not providing a custom date format, quoted numbers are treated as
   epoch milis/nanos
 * When not providing a custom date, quoted numbers are assumed to be handled
   by the pattern.

There is however no way to construct a pattern that handles both ISO dates
and epoch milis/nanos. This feature toggle allow numeric strings to be read
as numeric timestamps.

Fixes #263

datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserializer.java

@@ -343,10 +343,11 @@ protected T _fromString(JsonParser p, DeserializationContext ctxt,
             //   handled like "regular" empty (same as pre-2.12)
             return _fromEmptyString(p, ctxt, string);
         }
-        // only check for other parsing modes if we are using default formatter
+        // only check for other parsing modes if we are using default formatter or explicitly asked to
         if (_formatter == DateTimeFormatter.ISO_INSTANT ||
             _formatter == DateTimeFormatter.ISO_OFFSET_DATE_TIME ||
-            _formatter == DateTimeFormatter.ISO_ZONED_DATE_TIME) {
+            _formatter == DateTimeFormatter.ISO_ZONED_DATE_TIME ||
+            ctxt.isEnabled(DeserializationFeature.READ_NUMERIC_STRINGS_AS_DATE_TIMESTAMP)) {
             // 22-Jan-2016, [datatype-jsr310#16]: Allow quoted numbers too
             int dots = _countPeriods(string);
             if (dots >= 0) { // negative if not simple number

datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/ser/ZonedDateTimeSerTest.java

@@ -926,6 +926,18 @@ public void testCustomPatternWithAnnotations() throws Exception
         assertEquals(input.value.toInstant(), result.value.toInstant());
     }
 
+    @Test
+    public void testCustomPatternWithNumericTimestamp() throws Exception
+    {
+        String input = a2q("{'value':'3.141592653'}");
+
+        Wrapper result = MAPPER.readerFor(Wrapper.class)
+            .with(DeserializationFeature.READ_NUMERIC_STRINGS_AS_DATE_TIMESTAMP)
+            .readValue(input);
+
+        assertEquals(Instant.ofEpochSecond(3L, 141592653L), result.value.toInstant());
+    }
+
     @Test
     public void testNumericCustomPatternWithAnnotations() throws Exception
     {