Add filename to the end of RustFmtRange command

[flip1995]

Addresses #156

The --file-lines argument seems to need the filename at the end of the command.

With this change the :RustFmtRange command now works on all selections where executing the rustfmt command in the terminal would work.

For the following code executing

rustfmt --write-mode=overwrite --file-lines '[{"file": "example.rs", "range": [0, 2]}]' example.rs

and highlighting the first two lines and run :RustFmtRange does the same:

struct A {
t: i64,
}

mod foo {
    fn bar() {
                         // test
                             let i = 12;
                                 // test
    }
    // aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    fn baz() {
        let j = 15;
    }
}

The :RustFmtRange command is still limited by rust-lang/rustfmt#1514

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @chris-morgan (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[flip1995]

whoops this was already done in #185

Sorry for duplicate, but please merge the other PR.