Feature/publication themes

[adamrobinson361]

Background

To resolve #29.

Notes

1. Rather than using use_style I've found that the best way is to make custom document output types. I've made functions for national_statistic and official_statistic that can be referred to as output_format in Rmarkdowns via dfeR::national_statistic and dfeR::official_statistic. You can also get a template with all the markdown examples via File > New file > R Markdown > From Template.
2. I've removed yams as a dependency as its been messing up installs. it itself is a dependency of rmarkdown so that should cover us.
3. I've left out the other template for now that we use as want to have more of a think about it with the cover pages and whatnot.

QA

1. Check the code looks ok. Its just using the word_document function so should be fine
2. Check that the word doc template outputs look ok. I slightly edited them after you changed them as they were still a bit out of sync.
3. Check documentation is clear
4. Anything else that you think is needed to prepare for use. Hopefully these will be useful to data group.

[lauraselby]

I fixed a typo and added the example plot as discussed, though had to make a few other edits to keep the template output to one page after knitting (reduced table output to 5 rows rather than 10).

I've added ggplot to the imports, grateful if you could review before I approve.

[adamrobinson361]

Thanks @lauraselby - happy with all the changes if you are good to approve.