Add forcats vignette

[robinsones]

partially solves #149, making another branch with README changes.

[hadley]

This looks great! My only concern is the number of plots — because the vignette is built and included in the package, this will greatly increase the package size. I know that they're the most effective way of displaying the output but maybe you could use text output in a few more places?

[robinsones]

I cut it down to three graphs, showing fct_infreq() and fct_reorder(). I also added an example of changing the other_level argument.

[hadley]

Thanks @robinsones! Can you please check the website in ~10 minutes to make sure it's now also shown there?

[robinsones]

Unfortunately the website has not updated:

[hadley]

Ooops: https://travis-ci.org/tidyverse/forcats/jobs/481793301#L3141. I forgot about this - you'll need to load dplyr/ggplot2 explicitly and add them to suggests.