Make partial_plot use NSE

[mattmills49]

I'd like the partial_plot function to use non-standard evaluation so the function call doesn't have to use character values. Both for the code to be cleaner and because I want to learn how NSE works.

For example here is how the function currently works:

test_model <- mgcv::gam(mpg ~ s(hp), data = mtcars)
partial_plot(test_model, "hp")

But I'd like it to drop the quotes

partial_plot(test_model, hp)