Add non-github repo for graph-tool (#60)

Author: jbednar

tools/tools.yml

@@ -265,6 +265,10 @@
       site: https://clustergrammer.readthedocs.io
       badges: pypi, site
 
+    - repo: skewed/graph-tool
+      site: http://graph-tool.skewed.de
+      badges: pypi, site
+
 - name: Large-data rendering
   intro: Tools for rasterizing/aggregating data before visualization, which can make rendering faster, allow larger datasets, or make it feasible to use remote datasets (with server-side rendering).
   packages: