[MRG+1] add ftp to the link extractor scheme list

[nctl144]

This might be the solution to the issue mentioned in #1381. I thought it might be a good idea to solve it :). I added ftp to link extractor so it recognizes ftp links. Please let me know if there's anything that I can do to improve this PR 😄

[codecov]

Codecov Report

Merging #3152 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3152   +/-   ##
=======================================
  Coverage   82.11%   82.11%           
=======================================
  Files         228      228           
  Lines        9588     9588           
  Branches     1385     1385           
=======================================
  Hits         7873     7873           
  Misses       1456     1456           
  Partials      259      259

Impacted Files	Coverage Δ
scrapy/linkextractors/__init__.py	100% <100%> (ø)	⬆️

[kmike]

Thanks @nctl144! I think it is a right thing to do. We have some issues with processing ftp links (see #2505), but I think it still makes sense to extract them in link extractors.

[nctl144]

Thank you @kmike 😄