Change tfdoc pre-commit hook script to use while read

[rosmo]

Use read line instead of xargs. While probably a bit slower, this works reliably on Linux and OS X. (BSD and GNU xargs are unfortunately with different arguments) Also added option to set Python binary (eg. export PYTHON=python3). pre-commit.com hooks stash uncommitted changes, so patching it locally isn't really a good option.

On OS X:

Regenerate README.md with tfdoc.py........................................Failed
- hook id: cff-readme
- exit code: 1

xargs: command line cannot be assembled, too long

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass