Add a hook for specifying extra extras (e.g. uvloop) when running the buildout

buildout.cfg

@@ -4,6 +4,7 @@ parts =
     test
     scripts
 versions = versions
+extra =
 
 [versions]
 
@@ -11,7 +12,7 @@ versions = versions
 [test]
 recipe = zc.recipe.testrunner
 eggs =
-    ZEO [test]
+    ZEO [test${buildout:extra}]
 initialization =
   import os, tempfile
   try: os.mkdir('tmp')
@@ -21,6 +22,5 @@ defaults = ['--all']
 
 [scripts]
 recipe = zc.recipe.egg
-eggs =
-    ZEO [test]
+eggs = ${test:eggs}
 interpreter = py