Add plugin version in es-plugin.properties

Closes #59.

pom.xml

@@ -110,6 +110,15 @@
     </dependencies>
 
     <build>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+                <includes>
+                    <include>**/*.properties</include>
+                </includes>
+            </resource>
+        </resources>
         <plugins>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>

src/main/resources/es-plugin.properties

@@ -1 +1,2 @@
 plugin=org.elasticsearch.plugin.mapper.attachments.MapperAttachmentsPlugin
+version=${project.version}