Add a plugin for Syntastic.

It's pretty bare bones support. It basically just checks if the syntax of a Rust file is correct.

There is a more useful `--no-trans` option that can be given to `rustc`, but it doesn't work well as a default since it needs to be given the crate root file (otherwise it will generate false errors). This behavior can be achieved by overriding some options in your `.vimrc` (in my case, I put them in my project specific `.vim` file):

```vimscript
let g:syntastic_rust_rustc_fname = "src/lib.rs"
let g:syntastic_rust_rustc_args = "--no-trans"
```