feat: add linear loudnorm option to set lra up to target, then keep input lra

[mjhalwa]

This aims to fix #227

• Adds Option --keep-lra-above-loudness-range-target that may be used instead of --keep-loudness-range-target.
  the new option --keep-lra-above-loudness-range-target sets loudness range target (e.g. the default 7.0) for all input files that already have an LRA of less than the loudness range target. But behaves the same as --keep-loudness-range-target for higher input LRAs

  This automatically solves an issue with --keep-loudness-range-target for input files with LRA less than 1, where ffmpeg only allows LRA within the range [1, 50].

• The usage of both flags together is restricted with raising an error, as for LRA lower than the set --loudness-range-target one flag would else need to be silently ignored.

I hope it is ok, that I added a description of this option to the argparser and the ReadMe.

I tried to pick up your style. Please feel free to correct anything that does not fit your point of view. I will correct it. In case of necessary corrections please tell me if I should force push the changes to the same commit (in order to not create multiple commits) or if I should simply do another commit.

Hope you like it :)

[slhck]

Merged with minor wording changes and black formatting applied. Thanks a lot for your contribution!