xplatform path lookup, update readme

[kapilt]

use shutil.which for cross platform path lookups.

update readme docs on recording mode

closes #40

[codecov]

Codecov Report

Merging #41 (61382cf) into master (17bb7e8) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   92.64%   92.56%   -0.09%     
==========================================
  Files           8        8              
  Lines         462      457       -5     
==========================================
- Hits          428      423       -5     
  Misses         34       34              

Impacted Files	Coverage Δ
pytest_terraform/tf.py	96.21% <ø> (-0.09%)	⬇️
pytest_terraform/plugin.py	96.77% <100.00%> (+0.10%)	⬆️

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[kapilt]

coverage decrease is because we're deleting covered code.