Support to resolve paths in ENV['PATH'] with "~"

Hi, I would like to use `ptools` in one of my projects.

For testing I need to mock the home directory. To make this work. I use "~" and a modified HOME-variable. The "~" can be expanded by `File.expand_path` based on `ENV['HOME']`. Later on there's a test if an executable exists in "~/bin" using ptools's `which`. Unfortunately that fails because `ptool` cannot resolve paths in `ENV['PATH']`beginning with "~".

I added this functionality and a test for that.