Feature: implement --apparent-size

[imsnif]

Right now, diskonaut uses blocks in order to count file size. This means it counts how much space a file takes on the hard-drive, rather than how big that file is. These two can be different when (for example) the filesystem uses compression, and so the actual size taken on the hard-drive would be smaller than the file size.

Since differentiating them, while useful, is very much an edge-case - let's implement an --apparent-size flag rather than displaying both of them on-screen and possibly providing too much information to most users. This will also help us run tests predictably on platforms that use file-system compression (as discussed here: #50).

[imsnif]

Released in 0.9.0