Make discrimination and distribution functions

[trinker]

Part of the process of modeling is checking the discrimination and distribution. Making the following functionality will be useful and each will return a data frame with a plot method, tag_discrimination, tag_distribution, tag_co_occurrence with the last getting revisions to look more like the other 2. Maybe summary methods for some as well

Based on:

## Discrimination
model %>%
    as_terms() %>%
    plot_freq(size=3) +
    xlab("Number of Tags")

## Category counts
model %>%
    as_terms() %>%
    plot_counts() +
    xlab("Tags")