add support for Pipfile.lock

[nim65s]

Hi,

This PR adds support for Pipfile.lock (https://github.com/pypa/pipfile), with a new -p option for the CLI, a really simple read_pipfile util inspired from the existing read_requirements, and a unit test.

Another option to add this suppport (and support other formats), would be to refactor read_requirements to let dparse get the requirements from multiple formats, maybe with a CLI switch with dparse.filetypes as choices.

[codecov[bot]]

Codecov Report

Merging #216 into master will decrease coverage by 0.12%. The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   74.28%   74.15%   -0.13%     
==========================================
  Files           7        7              
  Lines         350      356       +6     
==========================================
+ Hits          260      264       +4     
- Misses         90       92       +2

Impacted Files	Coverage Δ
safety/util.py	62.9% <100%> (+1.88%)	:arrow_up:
safety/cli.py	54.87% <40%> (-0.82%)	:arrow_down:

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[GhostofGoes]

Why have a separate argument for Pipfile instead of using -f and checking the filename?

[nim65s]

Because we don't have access to the filename when using stdin.

But what we could do would be to check if the file contains valid JSON data, and in this case assume it is a Pipfile.lock. eg: https://github.com/nim65s/safety/commit/575636a90506fe0badbba00fd6f869e970bd5d12

I'll let the maintainers of this project choose :)

[alvarolmedo]

+1