Adding ICD-9 and ICD-10 conversions

[dapritchard]

My group plans to implement a conversion procedure between ICD-9 and ICD-10 codes in R based on the codes found here: http://www.nber.org/data/icd9-icd-10-cm-and-pcs-crosswalk-general-equivalence-mapping.html. It seems like this package would be a natural location for such functionality -- would you consider a pull request if I added such a feature? Do you have any preferences what the API would look like?

[wenjie2wang]

I would like to mention the existing implementation in the touch package: https://hub.wenjie-stat.me/touch/reference/icd_map.html. In fact, I sent emails to Jack and suggested to import touch::icd_map() to the icd package as the engine function for the ICD code conversion/crosswalk in Oct, 2018.

[dpritchLibre]

Note that this conversation is continued in a PR for this issue at #190