Store or calculate char_offsets, adding the offset of the doc end

def _get_char_offsets(self, char_offsets):
        """Store or calculate char_offsets, adding the offset of the doc end"""
        if char_offsets:
            char_offsets = char_offsets
            char_offsets.append(len(self.text))
        else:
            char_offsets = np.zeros(len(self.tokens) + 1)
            for i, tok in enumerate(self.tokens):
                # Add 1 to account for the spaces between tokens
                char_offsets[i + 1] = char_offsets[i] + len(tok) + 1
            char_offsets[-1] = len(self.text)
        return np.array(char_offsets)