rename package to gitignorer

pathspec just didn't seem right, even though i'm now ignoring the
chefignore support.  i have no interest in that.  what i do
want is to load the gitignore (system, global and project/dir) into
an object (struct) and keep using it over and over.
filter.go: first step to a reusable filter for gitignore

don't want to reload the file each time we want to check a file.
want it to be efficient
support a hierarchy of .gitignores

use the user's ~/.gitignore and also the first .gitignore in the directory
hierarchy above the current dir.  and, provide a method to manage
a stack of .gitignore as you move thru directories