Emit to the pip-compile's header an original command

[atugushev]

Uses sys.argv to emit an original command. Closes #726.

Changelog-friendly one-liner: Emit to the pip-compile's header an original command

Contributor checklist

• Provided the tests for the changes.
• Requested a review from another contributor.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[blueyed]

Restarted Travis builds - there appears to have been a temporary failure with downloads for py34/py35/pypy/pypy3.

[codecov]

Codecov Report

Merging #733 into master will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #733      +/-   ##
==========================================
+ Coverage   85.57%   85.81%   +0.24%     
==========================================
  Files          31       31              
  Lines        2128     2122       -6     
  Branches      310      305       -5     
==========================================
  Hits         1821     1821              
+ Misses        236      233       -3     
+ Partials       71       68       -3

Impacted Files	Coverage Δ
tests/test_cli.py	97.92% <100%> (-0.02%)	⬇️
tests/test_writer.py	100% <100%> (ø)	⬆️
piptools/writer.py	94.05% <100%> (+4.87%)	⬆️

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[atugushev]

@blueyed yes, thank you! It seems everything is okay now.

[blueyed]

Great. Will let you / somebody else merge this.

[vphilippon]

LGTM 👍

[vphilippon]

pip-tools v3.4.0 released