[MRG+1] Add support for executing scrapy using -m option of python

[luzfcb]

This pull-request enable execute scrapy from python module.

python -m scrapy

On windows, this probably works, too. Although I did not test because I do not have Windows

py -3 -m scrapy

[codecov-io]

Codecov Report

Merging #2740 into master will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #2740      +/-   ##
==========================================
- Coverage   84.75%   84.73%   -0.02%     
==========================================
  Files         162      163       +1     
  Lines        9155     9158       +3     
  Branches     1358     1359       +1     
==========================================
+ Hits         7759     7760       +1     
- Misses       1145     1146       +1     
- Partials      251      252       +1

Impacted Files	Coverage Δ
scrapy/__main__.py	33.33% <33.33%> (ø)

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[kmike]

+1 to merge, I also wanted to add this feature.

[luzfcb]

For reference, I've learned to do this based on this pull-request, done by one of Python's core commiters

cookiecutter/cookiecutter#788