Add github source and buildspec extraction to generate-pipeline

[jamesls]

This PR adds two options to the generate-pipeline command:

• The ability to specify a github source via --source github. CFN template params are injected that require you to specify the owner and repo name when this option is provided.
• The ability to write the CodeBuild buildspec file to a separate file. The default behavior is to include the contents as part of the cloudformation template.

[kyleknap]

General change looks fine to me. Could you elaborate on the benefits of defining the buildspec.yml outside of the cloudformation template? Is it so you can update the CodeBuild job out of band and not have to do an entire CloudFormation deploy?

[jamesls]

@kyleknap Yep exactly. It means that the people that have access to the source repo can now control how the codebuild process happens (vs. whoever manages the cfn template, which could be either separate teams or at least separate roles). I'll update the help message with this.

[codecov-io]

Codecov Report

Merging #619 into master will increase coverage by 0.02%.
The diff coverage is 97.87%.

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
+ Coverage   94.94%   94.96%   +0.02%     
==========================================
  Files          18       18              
  Lines        3203     3236      +33     
  Branches      417      421       +4     
==========================================
+ Hits         3041     3073      +32     
  Misses        110      110              
- Partials       52       53       +1

Impacted Files	Coverage Δ
chalice/cli/__init__.py	84.84% <100%> (+0.55%)	⬆️
chalice/pipeline.py	99.1% <97.5%> (-0.9%)	⬇️

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[jamesls]

@kyleknap ready to look at again.

[kyleknap]

Thanks. 🚢