Feature/plugin system

Adds a plugin system. See issue 13.

Plugins can be executed at these times:

• pre_install, post_install, pre_uninstall, post_uninstall, pre_build, post_build, init, exit.

Three example plugins added:

• post_install/wings3d/00_install_missing_deps.sh (installs needed deps that PPM/Pkg don't pick up by default)
• post_install/gimp/50_install_plugins.sh (only runs if --ask was given, asks to install gimp plugins)
• post_install/00_fix_deb_packages.sh (runs only when a .deb package is installed.. does nothing, just an example)

Screenshots:

Wings 3D:

Gimp:

:)