Add ability to run without debugging

[DonJayamanne]

Fixes #796

[DonJayamanne]

Restarting Azure CI

[codecov-io]

Codecov Report

Merging #797 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
- Coverage   59.64%   59.62%   -0.03%     
==========================================
  Files          27       27              
  Lines        3878     3881       +3     
==========================================
+ Hits         2313     2314       +1     
- Misses       1565     1567       +2

Impacted Files	Coverage Δ
ptvsd/debugger.py	90% <50%> (-10%)	⬇️

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[DonJayamanne]

@karthiknadig
Please could you merge this PR.
Azure CI is failing due to some flaky tests (i've restarted CI twice, and it fails in different places each time).
I.e. unrelated to the PR.