Set tmpRoot on tree if tmpRoot provided to builder

I know this needs tests, but I wanted to start the discussion. This is for #202.
Let's default `tmpRoot` to `tmp` (the current default value) if `options.tmpRoot` was not specified.  As it stands here if a plugin expects `tmpRoot` to be set but the user did not provide one when creating the `Broccolo::Builder` instance the plugin would error.