add --list option for borg create

[ThomasWaldmann]

like --stats enables statistics output, --list enables the file/dirs list output.

[codecov-io]

Current coverage is 83.38%

Merging #553 into master will not affect coverage as of dcf3e88

@@            master    #553   diff @@
======================================
  Files           13      13       
  Stmts         3917    3919     +2
  Branches       678     678       
  Methods          0       0       
======================================
+ Hit           3266    3268     +2
  Partial        197     197       
  Missed         454     454       

Review entire Coverage Diff as of dcf3e88

---

Uncovered Suggestions

1. +0.39% via borg/helpers.py#781...795
2. +0.31% via borg/archiver.py#371...382
3. +0.28% via borg/key.py#407...417
4. See 7 more...

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