[vis] render line breaks in TableViz (#2118)

* convert line breaks to br tags in table vis

* use css!

superset/assets/visualizations/table.css

@@ -23,3 +23,7 @@
 table.table thead th.sorting:after, table.table thead th.sorting_asc:after, table.table thead th.sorting_desc:after {
     top: 0px;
 }
+
+.like-pre {
+  white-space: pre;
+}

superset/assets/visualizations/table.js

@@ -72,6 +72,9 @@ function tableVis(slice, payload) {
       if (c === 'timestamp') {
         val = timestampFormatter(val);
       }
+      if (typeof(val) === 'string') {
+        val = `<span class="like-pre">${val}</span>`;
+      }
       return {
         col: c,
         val,