Add support for comments in .jshintignore

Following the assumption that a `.jshintignore` file should follow the `.gitignore` syntax (although [this is not documented](https://github.com/jshint/jshint/issues/1745)), adding proper support for comments (described in the second bullet [here](https://www.kernel.org/pub/software/scm/git/docs/gitignore.html#_pattern_format)). 

Most comments will likely not cause issues currently because they won't match on any files. However, if a comment contains `**`, and likely various other combinations of special regex characters, jshint fails with `SyntaxError: Invalid regular expression: ...` at https://github.com/jshint/jshint/blob/681b406918e96044be3375e389ddbe931740a804/src/cli.js#L232.