Add an option to exclude a directory

[ZNielsen]

Hey there! Love dust and find it very useful. I would like to have an option to exclude directories from the search/listing though. Is there a way to do this (I don't see it in the help menu)? If not, I think adding a -X / --exclude option would be useful. I can do the work if you think it is technically feasible. Any thoughts?

[bootandy]

If you want to add it I will merge the code.

You probably want to pass a 'excluded list down to here': https://github.com/bootandy/dust/blob/master/src/utils/mod.rs#L112 and then ignore things in the list.

Also watch out for this bug: #44 which shows up on small test directories. I'll fix soon that soon, but I don't want you thinking you've broken it.

Do you think when we do this that we should also add an option for'.' ie: -X . to exclude all hidden files (ie .*) ?