Retain factor levels (even if unseen)

[jonocarroll]

Closes #39.

Added as a branch of the dt_paste PR because I wasn't sure how it would interact (diff will reduce significantly once that PR is merged).

Extracts (all) factor levels and pastes them into the factor (not as.factor) call.

head(iris) %>% df_paste()
data.frame(
   Sepal.Length = c(5.1, 4.9, 4.7, 4.6, 5, 5.4),
    Sepal.Width = c(3.5, 3, 3.2, 3.1, 3.6, 3.9),
   Petal.Length = c(1.4, 1.4, 1.3, 1.5, 1.4, 1.7),
    Petal.Width = c(0.2, 0.2, 0.2, 0.2, 0.2, 0.4),
        Species = factor(c("setosa", "setosa", "setosa", "setosa", "setosa",
                           "setosa"), levels = c("setosa", "versicolor",
                           "virginica"))
)

Please test thoroughly before merging.

[jonocarroll]

Should the levels part start on a new line? Indenting that properly may be fiddly.

[jonocarroll]

This PR may now be out of date. I will confirm, but I think factor levels are now handled, no?