Add shiny app

[russHyde]

Prototype:

• App should run locally on user's computer
• They can point the app to a directory, package or file
• Dupree will run and a table of duplications will be created

Would like:

• Graph-based visualisation of duplications
• Nested-graph (blocks nested inside files) or circular (ordered blocks inside files on the perimeter with links between dups) visualisation

[russHyde]

Better to write this as a stand-alone repository until I've thought about security issues more thoroughly.

User's would upload files to the server,
The app would assess duplication within the file(s) [since based on lintr's code-parser, does this require the code to be ran?]
The files would be stored temporarily on the server [when should they be deleted?; should the results be cached between runs?]

If written for use on local-computer only using shinyFiles, the user would be able to navigate the file system and potentially see the contents of files (since dupree prints the contents of files in it's report) - if someone then installed dupree (and the associated app) on a webserver, would this pose a security risk, since the user could navigate to see secrets that are stored in the file system?