Deterministic output

[jyn514]

#74 fixed #56 in the sense that now all errors are always reported. However, the order of the errors is non-deterministic because deadlinks uses walkdir and rayon, which are non-deterministic.

Ordering errors within a file will probably not be too difficult - it will require checking the whole file before reporting errors, but since individual files don't take long to check I don't forsee much of a problem there. The tricky thing will be ordering the files themselves - a naive 'run all checks, then report all errors' will greatly slow down the time before the first report, because file directories can be (and often are) deeply nested.

cc @martin-t