Add snippet for the subject pattern in Jasmine

This snippet is helpful for writing Jasmine specs using the subject
pattern. See [recent blog post about subject pattern][subject-blog-post]
by @trotzig.

Snippet taken from [@lencioni's dotfiles][lencioni-dotfiles].

[subject-blog-post]: https://medium.com/brigade-engineering/dry-up-your-jasmine-specs-with-a-subject-d941b888db97
[lencioni-dotfiles]: https://github.com/lencioni/dotfiles/blob/master/.vim/UltiSnips/jasmine.snippets