Support resuming iteration from saved end_cursor. Implements #226 (WIP)

[tonycpsu]

• Adds a cursor argument to the pages() method to begin iteration using a stored end_cursor from a previous request.
• Adds a -c / --cursor command-line argument to support downloading beginning at a given end_cursor value.

The latter isn't particularly useful yet without a way to either display cursors in the downloader output or better yet save them to the cache directory, but it demonstrates the functionality.

This is a work in progress, but I figured I'd show what I have now to see if it merits merging with some additional documentation and polish.

[codecov]

Codecov Report

Merging #316 (327c3d3) into master (282de89) will decrease coverage by 28.79%.
The diff coverage is 59.68%.

@@             Coverage Diff             @@
##           master     #316       +/-   ##
===========================================
- Coverage   79.83%   51.04%   -28.80%     
===========================================
  Files          16       26       +10     
  Lines         972     1538      +566     
  Branches      134      151       +17     
===========================================
+ Hits          776      785        +9     
- Misses        155      738      +583     
+ Partials       41       15       -26     

Impacted Files	Coverage Δ
instalooter/_impl.py	100.00% <ø> (ø)
instalooter/_utils.py	55.00% <ø> (-33.47%)	⬇️
instalooter/cli/__init__.py	23.72% <0.00%> (-38.99%)	⬇️
instalooter/cli/constants.py	83.33% <ø> (ø)
instalooter/pbar.py	100.00% <ø> (ø)
tests/test_pbar.py	100.00% <ø> (ø)
instalooter/looters.py	37.36% <31.81%> (-36.83%)	⬇️
tests/test_looter.py	62.50% <42.30%> (ø)
tests/test_login.py	46.51% <50.00%> (ø)
instalooter/pages.py	39.13% <57.14%> (-46.72%)	⬇️
... and 22 more

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