Add JavaScript style guide

https://trello.com/c/LQsUqhYU/179-support-javascript
I'm proposing this:
```ruby
style_guide = StyleGuide::JavaScript.new(...)
file = double(:file, content: "var blahh = 'blahh'")

violations = style_guide.violations_in_file(file)

expect(violations.first.messages).to eq "Missing semicolon"
expect(violations.first.line_number).to eq 1
```
This looks much easier for me to understand.