Source os/distro specific plugin files

I couldn't get OS/Distro specific plugin files to be sources without these commits, I might be doing something wrong, but the following is from the readme and I don't see how it would have worked without these changes:

If a directory ```~/.bashrc-plugin.myplugin.d``` exists, then the following files would be sourced as well (if they exist):

1. ```~/.bashrc-plugin.myplugin.d/common/plugin.conf```
2. ```~/.bashrc-plugin.myplugin.d/linux-common/plugin.conf```
3. ```~/.bashrc-plugin.myplugin.d/linux-debian/plugin.conf```
4. ```~/.bashrc.home.d/myplugin.conf```