Add --no-allow-unsafe option to compile

[jdufresne]

In future versions of pip-tools, the --allow-unsafe behavior will used
by default. Adding the --no-allow-unsafe option now allows users to
continue using the old behavior in a backward compatible way.

Refs #989

Contributor checklist

• Provided the tests for the changes.
• 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).

[codecov]

Codecov Report

Merging #1265 (f044a25) into master (bc9695c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1265   +/-   ##
=======================================
  Coverage   99.66%   99.66%           
=======================================
  Files          37       37           
  Lines        2980     2980           
  Branches      331      331           
=======================================
  Hits         2970     2970           
  Misses          5        5           
  Partials        5        5           

Impacted Files	Coverage Δ
piptools/scripts/compile.py	100.00% <ø> (ø)
tests/test_cli_compile.py	100.00% <ø> (ø)
tests/test_utils.py	100.00% <ø> (ø)

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[atugushev]

LGTM 👍