Add `funding` to package.json

[darcyclarke]

• Implement npm i "nag" at the end of output
  ex. 23 packages are looking for funding. Run "npm fund" to find out more.
• Implement --no-fund flag to opt-out of the "nag" when installing
• Implement npm fund subcommand which lists out all the funding references
• Write tests for the "nag", --no-fund & the npm fund subcommand
• Document the subcommand functionality
• Document the schema change of package.json (can use/reference this spec once updated)
• Add a visual representation for the funding field/value on package pages
• Add funding to Pakument/Corgi objects: https://github.com/npm/validate-and-store it looks like we don't really need it, the reference implementation at npm repo is actually using the full metadata in which makes sense since we don't want to tax the entire registry just for users that are manually running the npm fund <package> command.