[RRFC] platform-specific configuration values

[isaacs]

Suggested in #36

Request for an RFC to explore having platform-specific config values, which is particularly relevant if configuration values would ride along with installed packages as implied by #59.

Implementation is relatively straightforward, but the following bikesheds need to be addressed:

• Fields we wish to filter on. Is it just process.platform, or are there others?
• Is a match/not-match syntax sufficient? So a platform: win32 section would provide config overrides for windows, and platform: !win32 would provide config overrides for non-windows. Would we need version comparisons, or other things if we're filtering based on node version etc?
• What happens if multiple configs match the current platform? If you set platform: !win32 and then later platform: darwin, do we prioritize the most specific, or is it order-dependent? (Note that order dependency is particularly fraught when dealing with JS objects, where key order can be munged without realizing it.)

Straw proposal to go from:

; in an ini-formatted .npmrc file
script-shell = /bin/sh

[platform: win32]
script-shell = c:\Program Files\git\bin\bash.exe

[platform: linux]
script-shell = /bin/bash

{
  "name": "package-json-with-configs",
  "npm": {
    "script-shell": "/bin/sh",
    "platform: win32": {
      "script-shell": "c:\\Program Files\\git\\bin\\bash.exe"
    },
    "platform: linux": {
      "script-shell": "/bin/bash"
    }
}

[mikemimik]

In the context of there being "platform specific" config (generally speaking), then #28 seems to be tangentially related.