Added __y_label_trim as available var in link

Fix #147 . Flux (BETA) adds a blank space on `_y_label` and it breaks
with link built when using the var.

This PR add a new variable to be used on link and removes leading and
trailing characters from `__y_label`

dist/tooltip.js

@@ -215,6 +215,9 @@ System.register(["d3", "jquery", "lodash"], function (_export, _context) {
               };
               scopedVars["__y_label"] = {
                 value: yLabel
+              };
+              scopedVars["__y_label_trim"] = {
+                value: yLabel.trim()
               }; // Grafana 7.0 compatible
 
               scopedVars["__url_time_range"] = {