[WIP] Support Cargo

Work in progress. I would like to get early feedback from all of you.

This PR add the support of Cargo to Rust Linter. We try to find file `Cargo.toml` first, to decide which we should use it to lint the source file, `cargo` (if we found a `Cargo.toml`) or `rustc` (if not found). Both rustc and cargo's path are configurable.

We also need to improve [Linter repo](https://github.com/AtomLinter/Linter) to complete this work. Currently, blocked by https://github.com/AtomLinter/Linter/pull/290 and https://github.com/AtomLinter/Linter/pull/304. These two PRs of mine are under reviewing.

Closes #4
Probably should use more understandable names of config variables, e.g. ```rustcPath``` and ```cargoPath```