Automatically locate and use correct .jshintrc

Currently, the `.jshintrc` support requires setting a global config var pointing to the location of the `.jshintrc` file. This adds an extra configuration step (currently not documented in the README), and makes it difficult to work on multiple different projects with different `.jshintrc` files.

Ideally, each time `flymake` is initialized on a new buffer, it would use the Emacs function `locate-dominating-file` to automatically find the nearest `.jshintrc` file in containing directories, and automatically use that `.jshintrc` for that buffer.