Add support for Alpine Linux /lib/apk/db/installed (Resolves #72)

[cmaritan]

Add support for Alpine APK Installed Database V2 ( resolves #72 ):

• /lib/apk/db/installed

Reference: https://wiki.alpinelinux.org/wiki/Apk_spec

I tried to follow similar lockfile parsers (e.g. requirements.txt). Added some tests, updated README.

[cmaritan]

Hello @G-Rath,

thank you for your precious feedback, I'm learning a lot in contributing to this project. I updated the PR and think that all of your feedbacks have been added. Let me know.

Thank you very much. Regards.

[another-rex]

/gcbrun

[another-rex]

/gcbrun

[another-rex]

/gcbrun