PR-URL: npm#246
Credit: @kemitchell
Close: npm#246
Reviewed-by: @ruyadorno

Thanks @kemitchell for providing the initial work that served as a base
for `npm fund`, its original commits messages are preserved as such:

- support: add support subcommand
- support: fix request caching
- support: further sanitize contributor data
- doc: Fix typo
- support: simplify to just collecting and showing URLs
- install: improve `npm support` test
- install: drop "the" before "projects you depend on"
- doc: Reword mention of `npm support` in `package.json` spec

This commit introduces the `npm fund` command that lists all `funding`
info provided by the installed dependencies of a given project.

Notes on implementation:

- `lib/utils/funding.js` Provides helpers to validate funding info and
return a tree-shaped structure containing the funding data for all deps.
- `lib/fund.js` Implements `npm fund <pkg>` command
- Added tests
  - `npm install` mention of funding
  - `npm fund <pkg>` variations
  - unit tests for added `lib/utils` and `lib/install` helpers
- Added docs for `npm fund`, `funding` `package.json` property
- Fixed `lib/utils/open-url` to support `--json` config
- Documented `unicode` on `npm install` docs

Refs: https://github.com/npm/rfcs/blob/2d2f00457ab19b3003eb6ac5ab3d250259fd5a81/accepted/0017-add-funding-support.md