fixes for vis_expect

[njtierney]

Need to add:

• % of cases that the expectation is TRUE (a la vis_miss)
  - [ ] Maybe add in the % of cases true in each column?
• Tests for expect_frame
• Flesh out documentation for expect_frame
• Add expect_frame into vignette (Along with vis_cor()

[njtierney]

Here is the current behaviour (as of commit be1fdf6)

library(visdat)
dat_test <- tibble::tribble(
            ~x, ~y,
            -1,  "A",
            0,  "B",
            1,  "C"
            )

vis_expect(dat_test,
           ~ .x == -1)

vis_expect(airquality,
           ~ .x == 5.1)

Created on 2018-06-04 by the reprex package (v0.2.0).

[njtierney]

At the moment I am deciding to leave out the percentages of expectations that are TRUE in each column, I think you would be better off getting this kind of information from a dataframe summary method - as discussed in #83

I am going to keep the NA values as grey for the moment, but these might need to change in the future.