Add contrib mode, and rename 'check' to 'needs'

[grega]

Discover and parse "contributor"-focused docs (ie. CONTRIBUTING.md, DEVELOPMENT.md, DEVELOPERS.md, and HACKING.md) alongside the README.

Contributor docs appear in all modes with a file separator, and hdi contrib filters to only those files.

Renames the check / c subcommand to needs / n, freeing the c alias for contrib (slightly suboptimal, but feels like the best compromise in order to maintain the most obvious single-char aliases).

---

Test using:

./hdi test/fixtures/node-express

And:

./hdi c test/fixtures/node-express

And for completeness, the renamed check -> needs command:

./hdi n test/fixtures/node-express
./hdi n test/fixtures/java-spring
./hdi n test/fixtures/rust-cargo

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Considerations:

• A keyboard shortcut to jump between files? (similar to current tabbing, but between file headers)
• Preference to always show contrib output at the top, rather than under the parsed readme output?
  • Also maybe only show contrib output?
• Not convinced more vertical stacking is the way to solve this (eg. there's going to be a lot of content if we're parsing a README.md, CONTRIBUTING.md and a DEVELOPMENT.md, for example), maybe there needs to be some sort of "tabbed" (visually) interface
  • Perhaps this is fine for v1...
  • There could also be duplication between these files, but can de-dupe to some extent
• ...

[grega]

[maxelkins]

Will check this out tomorrow!