Make nltk.download quiet

[Guts]

It avoids output like this in upper-level tools (like a CLI using lunr for my use case):

[nltk_data] Downloading package stopwords to /home/guts/nltk_data...
[nltk_data]   Package stopwords is already up-to-date!

Since NLTK does not expose any log level and use a "simple" print, there is no other way.

[codecov-commenter]

Codecov Report

Merging #123 (8837a8d) into master (57b9950) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #123   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files          23       23           
  Lines        1096     1096           
  Branches      200      200           
=======================================
  Hits         1080     1080           
  Misses          9        9           
  Partials        7        7           

Impacted Files	Coverage Δ
lunr/languages/__init__.py	100.00% <100.00%> (ø)

[yeraydiazdiaz]

Thanks!