Add Julia comment style

[sorairolake]

Add the comment style for Julia.¹ This supports both single-line comments and multi-line comments.

Also, single-line multi-line comments cannot be parsed with CommentStyle.parse_comment(), so I changed this to run CommentStyle._parse_comment_multi() first.

Footnotes

1. https://docs.julialang.org/en/v1/base/punctuation/ ↩

[carmenbianca]

Awesome PR. I touched this up a little. I removed the tests because the tests are already generically done for all comment styles. The explicit tests for some other styles are a relic from before I made the tests generic, and they're kept for ease of debugging when something goes horrifically wrong.

Thanks a lot @sorairolake !