Introduce liq_cue_skip and fix bugs

[t0mtaylor]

proposed v4.2.0 with new options:

Reason for change: To allow rerun of cue_file within a script over files sync'd and so it wont reprocess files that have already been processed and tagged

The added benefit of the liq_cue_skip tag also now stops the file metadata/tags being written again to the media file, which would cause the modified date to change when it doesn't need to. This causes a nightmare for bi-direction sync.

• Introduce skip arg and liq_cue_skip tag for files to avoid reprocessing by force, as current logic doesn't seem to work as expected, this is backwards compatible with previously processed files and not a breaking change. Files may require reprocessing one time to add the new tag where required.

• The liq_cue_file must be true when skip is enabled, to allow Azuracast to read the tags and avoid any more re-processing, - although when true Azuracast outputs:cue_file results override existing metadata because liq_cue_file=true tells us to. which means Azuracast may need to support the skip tag to be forced to just read the tags when this is true.

• fix Mutagen m4a python TypeError: encoding without a string argument on Mac when writing tags for m4a/mp4 files (should be string).

• Add debug flag to allow some additional messages when required

example command line, which includes write, replygain, skip (-p), debug (-z):

./cue_file "filename.mp3" -wrpz

force will still work if required, or you remove skip for existing checks

[t0mtaylor]

testing with original skip_analysis, which works as expected after a file was processed with liq_cue_skip and when skip arg mode is not enabled