* Canonicalize file paths within the map of file line ranges
* Forgot to run the tests - and of course, the formatting of the canonicalization change was off, but it's fixed now!
* Move imports to the top of the file, as per @nrc.
* Change `canonicalize_path_string` to return `Option<String>`, `None` indicating an error rather than an empty string
* `format!` is better than string concatenation...
* Change `canonicalize_path_string` to return `Result` rather than `Option`
This commit adds a type to represent lines in files, and adds it to the
`Config` struct. It will be used for restricting formatting to specific
lines.
Refs #434
