Allow specification of FITS suffixes in ImageFileCollection

[keflavich]

Our observatory writes .FIT files, and there appears to be no way at
present to read these with an ImageFileCollection.

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[codecov]

Codecov Report

Merging #727 (909b659) into master (9611aa9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #727      +/-   ##
==========================================
+ Coverage   95.49%   95.51%   +0.02%     
==========================================
  Files          30       30              
  Lines        3931     3949      +18     
==========================================
+ Hits         3754     3772      +18     
  Misses        177      177              

Impacted Files	Coverage Δ
ccdproc/image_collection.py	99.21% <100.00%> (+0.01%)	⬆️
ccdproc/tests/test_image_collection.py	98.88% <100.00%> (+0.01%)	⬆️

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[mwcraig]

@keflavich -- do you have time to add a test for this? If not, I can do that Monday and then merge. I'm not sure why your files are not being picked up since fit was already supposed to be one of the recognized extensions but I'm 👍 on this feature in any event.

[keflavich]

no time, sorry 🕐