Profiler: Add import and/or export options

[bvaughn]

Add an "export" option that dumps profiling data to some JSON format so that it can be shared.

Investigate adding an "import" option that can read in exported profiling data and display it. (Perhaps this could be done with a standalone version of the profiler, similar to the DevTools theme browser.)

[bvaughn]

React DevTools has been rewritten and recently launched a new version 4 UI. The source code for this rewrite was done in a separate repository and now lives in the main React repo (github.com/facebook/react).

Because version 4 was a total rewrite, and all issues in this repository are related to the old version 3 of the extension, I am closing all issues in this repository. If you can still reproduce this issue, or believe this feature request is still relevant, please open a new issue in the React repo: https://github.com/facebook/react/issues/new?labels=Component:%20Developer%20Tools