Update only a single album

[dsschult]

Following up on #508, this PR adds a cli flag for updating a single album.

One key difference compared to the ignore_directories setting is that this will add any albums necessary to create the breadcrumbs in the selected album, but not write them.

Note that I had to adjust how the album source tree is walked to top-down mode, so it wouldn't walk the whole tree if it didn't need to. This meant changing how to ignore albums, including editing the nomedia plugin.

I also added a count of the albums processed to the stats, as that helped with debugging. I can remove that if necessary.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (27e10d4) 88.72% compared to head (34bb74a) 89.01%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #511      +/-   ##
==========================================
+ Coverage   88.72%   89.01%   +0.29%     
==========================================
  Files          25       25              
  Lines        2066     2076      +10     
==========================================
+ Hits         1833     1848      +15     
+ Misses        233      228       -5     

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