CLI: Remove JSON support for .eslintignore (fixes #883)

See #883 

Now we need to skip empty lines when we read `.eslintignore`:
```
            text = fs.readFileSync(filePath, "utf8");
            exclusions = text.split("\n").filter(function (line) {
                return line.trim() !== "";
            });
```

Otherwise the following test fails:
```
    describe("when searching for exclusions in directory without .eslintignore and empty parent .eslintignore", function() {
        it("should not have any exclusions", function() {
            var configHelper = new Config();

            configHelper.cacheExclusions(path.resolve(__dirname, "..", "fixtures", "rules", "test"));

            assert.isUndefined(configHelper.exclusionsCache[path.resolve(__dirname, "..", "fixtures", "rules", "test")]);
        });
    });
```

Do you agree with the filter approach or you prefer a regex? 
It seems quite readable with the filter function.