Accept json format for '~/.jshintrc'.

This change modifies `jshint.vim` to accept `.jshintrc` format for `~/.jshintrc` in addition to the previous `/*jshint opt: val*/` format. This should fix issue #10.

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It would be an api-breaking change to stop accepting comment format for `~/.jshintrc`, but at the same time, it's a little confusing for `~/.jshintrc` to not be in the standard format, given the filename.