new formula: avfs

[AVFS](http://avf.sourceforge.net/) is a file system, which enables all programs to look inside archived or compressed files, or access remote files without recompiling the programs or changing the kernel. It does this by mounting `/` to `~/.avfs` and allow the user to `cd` into archives. For example, if there is a zip file on the desktop, the user can `cd ~/.avfs/Users/user/Desktop/foo.zip#`.