Option to flatten results

[kibertoad]

Currently when "recursive" option is true, results get nested inside directory objects. In many cases you don't actually need that tree structure and flattened result is more convenient to consume.
This should probably come with some mechanism to ensure there are no duplicate filenames.

[kibertoad]

@dougwilson If you approve of this change, I can produce a PR.

[dougwilson]

Sorry I missed this issue when you opened it. I was just looking through everything now that all your PRs are merged looking to see if there is anything pending before releasing a 3.0 now. This feature definitely makes sense. It sounds like a semver-minor, so won't hold up 3.0 for this, but you're welcome to make a PR if you want 👍.

I'm thinking that maybe for the conflicts maybe:

(a) concat the dir names together with some character (maybe configurable?) like _. If you use a character that is illegal in a file / dirname like / then that would prevent the possibility of duplicates.
(b) provide a function (could map be reused here?) that would given the parth parts in like an array or something (or just a string of the real relative path) and expect the user to return whatever they want it to be.