Some improvements to the Vim Cargo compiler file.

Looks like I made my previous PR a little too hastily. =)

This PR fixes a couple issues that I discovered with my previous revision:

1. Updated the errorformat to ignore "pointer lines" so that they don't show up in the output (with quickfix jumping, they're redundant and unnecessary).
2. Renamed a couple variables to be more in line with Cargo's terminology (`g:cargo_toml_name` should now be `g:cargo_manifest_name`).
3. Added support for errors reported with absolute paths (looks to be the case when compiling an executable instead of a library).
4. Most importantly, added support for errors reported while compiling a dependency. When building a Cargo package with local dependencies, if one of those dependencies failed to compile, the quickfix would be completely broken as it assumed that all errors were relative to the local manifest, or the closest Cargo.toml. With this update, it now pays attention to lines that end with `(file://<path>)`, and from then on adjusts all errors to be relative to `<path>`.

As a side note, that `<path>` output is somewhat broken on Windows. While `file:///home/damien/...` on *Nix is a valid URI, `file:///C:/Users/damien/...` on Windows is not, because `C:/` (or whatever the drive is) should take the place of the third slash which is *Nix's root, not be appended to it. I added a workaround for this in my script, but I figured I'd mention it to see if this is a bug in how Rust formats paths.
Semantically cleaner as `if !empty(m)`