Support new language: Julia

As mentioned in #527 by @marcusps, it would be nice to support the Julia language. Here it is!

I've checked the "Language contributor checklist" and confirmed that this passed the unit test.
But before completing the job, let me ask some questions.

1. To avoid misclassification with Matlab, I slightly modified the relevance of comment style of it. Is this permissible?
2. I want to escape expressions surrounded by '$(' and ')' in a string literal like Ruby or CoffeeScript, but the way of Ruby and CoffeeScript is not satisfactory to me because the escaped expressions are highlighted awkwardly. Is there any way to completely escape these expressions as if they are in normal code?

When there is any inappropriate manner in this pull request, please let me know.

Thanks.
We now support `Category` here, the list of them could be found on [demo page](https://highlightjs.org/static/demo/). It is not required, but you may want to add it.