Add support for retaining existing album gain via CLI argument

[maszaa]

Via -p/--preserve-album-gain CLI argument

[kepstin]

Hi @maszaa - thanks for the PR! I have a couple requests:

Please split out the commit for using virtualenv/pip into a separate PR, since it's not related to the album gain issue.

I'd like the option name to be more positive, and better describe what happens. Instead of "ignore album gain", I think "preserve album gain" would be better. This is because the option should keep any album gain tags that are already present without modifying them.

[maszaa]

Good points, I updated the PR