add Urban Institute theme
PR merged
from ...utils.theme import ThemeRegistry
-VEGA_THEMES = ['ggplot2', 'quartz', 'vox', 'fivethirtyeight', 'dark', 'latimes']
+VEGA_THEMES = ['ggplot2', 'quartz', 'vox', 'fivethirtyeight', 'dark', 'latimes', 'urbaninstitute']
class VegaTheme(object):
