Add example for ProphetModel

[julia-shenshina]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Related Issue

Closing issues

closes #118

[codecov-commenter]

Codecov Report

Merging #178 (4278e6c) into master (cadbd6e) will increase coverage by 0.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
+ Coverage   87.22%   87.78%   +0.55%     
==========================================
  Files          74       75       +1     
  Lines        3272     3438     +166     
==========================================
+ Hits         2854     3018     +164     
- Misses        418      420       +2     

Impacted Files	Coverage Δ
etna/models/prophet.py	98.18% <ø> (ø)
etna/pipeline/pipeline.py	99.25% <0.00%> (-0.75%)	⬇️
etna/metrics/__init__.py	100.00% <0.00%> (ø)
etna/metrics/utils.py	100.00% <0.00%> (ø)
etna/model_selection/backtest.py	97.60% <0.00%> (+0.03%)	⬆️
etna/models/sarimax.py	94.89% <0.00%> (+1.01%)	⬆️

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[martins0n]

👍