install: new command to install packages

new apkg install command was added using two new pkgstyle interface functions for installing packages using native distro package manager:

• install_custom_packages() to install native package files (default)
• install_distro_packages() to install packages from distro repos (enabled with -D/--distro-deps option)

It's possible to override target distro using -d/--distro as usual.

Packages to install can be passed as command arguments or using -f including stdin support with -, i.e.:

apkg build | apkg install -f -

-y/--yes option enables non-interactive mode.

Fixes: #26 (closed)