Workaround for intermittent crashing (bug

https://github.com/paulmillr/chokidar/issues/133)
Updated README.
Allowing to set the extension root dir (`EXTENSION_ROOT_DIR` env var).
And this is the workaround for the `chokidar` (fs watching lib) vs `grunt-crx` bug (https://github.com/paulmillr/chokidar/issues/133).