expose DIFFABLE_MATCHERS to allow customizing it
expose DIFFABLE_MATCHERS to allow customizing it

Summary:
expose DIFFABLE_MATCHERS to allow customizing it
this allow formatMatchFailure to use the custom chai-diff messages for any custom matchers:

eg: in a theoretical `test-framework-setup.js`:
```
// ...
const JasmineFormatter = require('jest-util/lib/JasmineFormatter');
JasmineFormatter.DIFFABLE_MATCHERS.toMatchSnapshot = true;
// ...

failureMessage = formatter.formatMatchFailure(res);
```

will show something like:
<img width="1417" alt="screen shot 2016-05-09 at 2 36 01 pm" src="https://cloud.githubusercontent.com/assets/87300/15115038/ce7a75ae-15f4-11e6-8c54-66bc67ee19cb.png">
Closes https://github.com/facebook/jest/pull/981

Differential Revision: D3288360

fbshipit-source-id: 7d5d9a4c6755ca60705bb511d0ec855b7764661c

cpojer/jest,Daniel15/jest,skovhus/jest,alvinsj/jest,razh/jest,bookman25/jest,sampepose/jest,ide/jest,blainekasten/jest,aaron-goshine/jest,skovhus/jest,razh/jest,facebook/jest,mpontus/jest,skovhus/jest,Daniel15/jest,facebook/jest,blainekasten/jest,bookman25/jest,blainekasten/jest,aaron-goshine/jest,cpojer/jest,ide/jest,sampepose/jest,aaron-goshine/jest,Daniel15/jest,ide/jest,mpontus/jest,bookman25/jest,cpojer/jest,mpontus/jest,facebook/jest,alvinsj/jest