central issue for "default extensions"

This issue encompasses anything that deals with "what the default extensions are/should be".

- #19 points out that `.json` is not included in the defaults.
- #137 points out that `require.extensions` is not used as the defaults
- https://github.com/substack/tape/issues/395 is related, but can be addressed with changes in `tape`, unrelated to whatever `resolve` does.
- (related but addressed: #150, #127, #90, #134)

There's some PRs out that are hoping to change the defaults:
- #138 dynamically reads `require.extensions` on every call
- #145 replaces the default of `.js` with a hardcoded list of the default `require.extensions` value

I'm opening this so I can close the other issues, and so any further discussion can happen here.

Please try to read all of the previous discussions before commenting.