Add gitpod config

[miso-belica]

This commit adds support for Gitpod.io, a free automated
dev environment that makes contributing and generally working on GitHub
projects much easier. It allows anyone to start a ready-to-code dev
environment for any branch, issue and pull request with a single click.

[codecov]

Codecov Report

Merging #138 into master will decrease coverage by 0.6%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
- Coverage   81.29%   80.68%   -0.61%     
==========================================
  Files          37       37              
  Lines        1775     1719      -56     
==========================================
- Hits         1443     1387      -56     
  Misses        332      332

Impacted Files	Coverage Δ
sumy/parsers/html.py	57.62% <0%> (-3.92%)	⬇️
sumy/parsers/plaintext.py	80.64% <0%> (-1.45%)	⬇️
sumy/models/dom/_document.py	91.66% <0%> (-1.2%)	⬇️
sumy/models/dom/_paragraph.py	92% <0%> (-0.86%)	⬇️
sumy/summarizers/_summarizer.py	93.54% <0%> (-0.4%)	⬇️
sumy/summarizers/lex_rank.py	94.68% <0%> (-0.37%)	⬇️
sumy/summarizers/edmundson.py	94.5% <0%> (-0.34%)	⬇️
sumy/summarizers/text_rank.py	95.38% <0%> (-0.33%)	⬇️
sumy/models/dom/_sentence.py	96.15% <0%> (-0.28%)	⬇️
sumy/summarizers/lsa.py	90.27% <0%> (-0.27%)	⬇️
... and 6 more

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