Give warning if overwrite sub-component of spec

[AliciaSchep]

Example:

vl_chart() %>%
    vl_add_data(url = "https://vega.github.io/vega-editor/app/data/population.json") %>%
    vl_calculate(calculate = "datum.sex == 2 ? 'Female' : 'Male'", 
                 as = "gender") %>%
    vl_filter("datum.year == 2000") %>%
    vl_encode(x = "age:O", y = "people:Q", color = "gender:N") %>%
    vl_stack_y("normalize") %>%
    vl_aggregate_y("sum") %>%
    vl_axis_y(title = "population") %>%
    vl_mark_bar() %>% 
    vl_axis_y(labelAngle = 90)

If you call something (like vl_axis_y in this case) twice, you overwrite the y axis definition. Should it potentially merge instead? At minimum, should give a warning!