A few slight tweaks to the Vim syntax file.

- Add fold support (NOT turned on by default)
- Highlight `::` by default
- Support the common `NOTE` as an important note
- Highlight `assert!` and `fail!` differently
- Don't highlight `deriving(...)` except in `#[...]`
Add a Vim filetype plugin with useful tweaks.

This improves things like doc comment handling when you press Enter and
making using `gf` or `<C-W>f` work on a `use x;` statement in the
current directory.
Do indentation properly in Vim.

The cases that this improves are clearly documented inside the
committed file, so this commit message is comparatively brief.
auto merge of #7665 : chris-morgan/rust/better-vim-support, r=cmr

General tweaks for Vim support, especially proper indenting.