Add replace(), __getitem__, and __iter__ to pipeline

[dhdaines]

This allows one to more easily find registered functions (such as the broken language trimmers) in a pipeline and replace them (with less broken ones). Fixes #155

Missing changelog

[yeraydiazdiaz]

Thanks again.

Could you add entries in the Changelog under "Unreleased"? The same applies to the other PRs.

Happy to merge when the pipelines pass.

[dhdaines]

Ah! I forgot to run tox! Fixed now.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.30435% with 4 lines in your changes missing coverage. Please review.

Project coverage is 95.95%. Comparing base (d07b60f) to head (66ad327).

Files with missing lines	Patch %	Lines
lunr/pipeline.py	86.66%	2 Missing ⚠️
tests/test_pipeline.py	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
- Coverage   96.02%   95.95%   -0.07%     
==========================================
  Files          48       48              
  Lines        3171     3217      +46     
==========================================
+ Hits         3045     3087      +42     
- Misses        126      130       +4     

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[dhdaines]

Should be good now - it made sense to add __iter__ as well, with the caveat that Pipeline is not a real python mapping due to the issue of function registration.