This program takes two flags - src and dst and copies files that are present in src but not in dst and files
that are a different size (I tried using hashes to determine whether a file is different, but it was painfully slow).
Also, folders that are named dont_mirror will be ignored.
There's also an optional c flag that turns on "cleaning mode". In this mode, every file and directory that is present
in dst but not in src will be deleted. (Files with different sizes will be left alone)
I use this program for my personal use, so it isn't the fastest thing ever written, but it can handle a few million files just fine.
To generate a binary run go build main.go