Allow customizing export column names

[Gallaecio]

Fixes #1008, fixes #3266

[codecov]

Codecov Report

Merging #3696 (892d339) into master (de0e2cc) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

❗ Current head 892d339 differs from pull request most recent head ec5cf3e. Consider uploading reports for the commit ec5cf3e to get more accurate results

@@            Coverage Diff             @@
##           master    #3696      +/-   ##
==========================================
- Coverage   88.71%   88.58%   -0.14%     
==========================================
  Files         162      162              
  Lines       10740    10764      +24     
  Branches     1834     1842       +8     
==========================================
+ Hits         9528     9535       +7     
- Misses        939      953      +14     
- Partials      273      276       +3     

Impacted Files	Coverage Δ
scrapy/settings/default_settings.py	98.76% <ø> (ø)
scrapy/core/downloader/handlers/ftp.py	98.38% <100.00%> (ø)
scrapy/core/downloader/webclient.py	94.65% <100.00%> (ø)
scrapy/exporters.py	100.00% <100.00%> (ø)
scrapy/extensions/httpcache.py	95.41% <100.00%> (ø)
scrapy/responsetypes.py	94.28% <100.00%> (+0.25%)	⬆️
scrapy/settings/__init__.py	93.44% <100.00%> (+0.37%)	⬆️
scrapy/utils/conf.py	93.75% <100.00%> (ø)
scrapy/utils/asyncgen.py	33.33% <0.00%> (-66.67%)	⬇️
scrapy/utils/spider.py	69.69% <0.00%> (-12.13%)	⬇️
... and 3 more

[Gallaecio]

@wRAR, @kmike: Should we consider this a potential candidate for 2.7?

[wRAR]

I'm fine with that.

[kmike]

Thanks @Gallaecio!