Added unified fetching proposal

proposals/fetching.md

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+# Unified Cordova Fetching Proposal
+
+### Current
+
+Three existing fetching implementations.
+
+1. Platform Fetching 
+    - Platform.js https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/cordova/platform.js#L258
+    - Lazy Load https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/cordova/lazy_load.js
+
+2. Plugin Fetching
+    - possiblyFetch https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/plugman/install.js#L104
+    - fetch.js https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/plugman/fetch.js
+    - registry.fetch https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/plugman/registry/registry.js#L123
+
+3. Template Fetching
+    - create.js https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/cordova/create.js#L167
+    - remote load https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/cordova/remote_load.js
+
+Both lazy load and remote load use gitclone.js https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/gitclone.js
+
+Currently, platforms and templates are cached at `~/.cordova/lib/npm_cache/PACKAGE`. On master, plugins are being cached to `~/.npm/PACKAGE`.
+
+### Proposal:
+
+* Create a new module named `cordova-fetch`. Fetches from npm, git and local paths. 
+* Start using system installed `npm` instead of packaging our own `npm` in `cordoba-lib`. We already check for `git` being installed, we should do the same for `npm`. We would use  `superspawn` to shell out to the system `npm`.
+* Stop using `.cordova/npm_cache/` and instead use `.npm/` for all of our `npm caching`. This will allow us to move away from passing custom config options to npm. 
+* Update existing fetching logic to use new `cordova-fetch` module.