New documentation snippets for Julia

I've added some snippets for writing documentation. There's also a new TODO snippet and a ternary operator snippet.
This format is not used in [JuliaLang/julia](https://github.com/JuliaLang/julia), since the code generally has very little documentation. However, [JuliaStats/DataFrames.jl](https://github.com/JuliaStats/DataFrames.jl) uses this documentation style across the entire codebase and I find it quite useful/clear.
The official Julia style guide does not say anything about documentation. There is a fairly [large discussion thread](https://groups.google.com/forum/#!msg/julia-users/-GggWwLk4Ks/nXDzNo1vWS8J) regarding a Julia style guide published by [John Myles White](https://github.com/johnmyleswhite/Style.jl), which suggests you should not make use of extensive comments for algorithms in your code, but rather write a separate documentation.