Add before_build option, that runs a shell command before 'pip wheel'

cibuildwheel/util.py

@@ -0,0 +1,11 @@
+
+def prepare_command(command, python, pip):
+    '''
+    Preprocesses a command by expanding variables like {python} or {pip}.
+
+    For example, used for the before_build option, where the user would
+    like to run a command like `python setup.py test`. If the command should run on
+    Python 3, the user could write `{python} setup.py test`. This command would expand
+    it out to python2 or python3 as appropriate.
+    '''
+    return command.format(python=python, pip=pip)