Returns a dictionary of keys with the same prefix.
            Compat with kr/env, lowercased.

            > xdg = env.prefix('XDG_')

            > for key, value in xdg.items():
                 print('%-20s' % key, value[:6], '…')
            config_dirs      /etc/x…
            current_desktop  MATE
            data_dirs        /usr/s…
            …

def prefix(self, prefix, lowercase=True):
        ''' Returns a dictionary of keys with the same prefix.
            Compat with kr/env, lowercased.

            > xdg = env.prefix('XDG_')

            > for key, value in xdg.items():
                 print('%-20s' % key, value[:6], '…')
            config_dirs      /etc/x…
            current_desktop  MATE
            data_dirs        /usr/s…
            …
        '''
        env_subset = {}
        for key in self._envars.keys():
            if key.startswith(prefix):
                new_key = key[len(prefix):]  # cut front
                new_key = new_key.lower() if lowercase else new_key
                env_subset[new_key] = str(self._envars[key]) # str strips Entry

        return Environment(environ=env_subset,
                           sensitive=self._sensitive,
                           blankify=self._blankify,
                           noneify=self._noneify,
                           writable=self._writable,
                          )