Fix comment indenting properly for Vim files.

Indentation now works correctly on subsequent lines of a multi-line
comment, whether there are leaders (` * `) or not. (Formerly it was
incorrectly doing a two-space indent if there was no leader.)

By default, this no longer puts a ` * ` leader on `/*!` comments, as
that appears to be the current convention in the Rust source code, but
that can easily be re-enabled if desired:

    let g:rust_bang_comment_leader = 1