Included $col as an option for the "Alias pattern" on influxdb

With this patch it's possible to use the column name as an identifier.
This can help when we have multiple columns as it would be possible to automatic generate a legend.
Also this would help in future when grafana could support the "AS" in the query builder.

public/app/plugins/datasource/influxdb/partials/query.editor.html

@@ -210,6 +210,7 @@
 			<ul>
 				<li>$m = replaced with measurement name</li>
 				<li>$measurement = replaced with measurement name</li>
+				<li>$col = replaced with column name</li>
 				<li>$tag_hostname = replaced with the value of the hostname tag</li>
 				<li>You can also use [[tag_hostname]] pattern replacement syntax</li>
 			</ul>