Use proper variable interpolation

I don't know why, but `env()` wasn't working, but `"$VARIABLE"` works, so I just switched the variable expansion method.

Fix #4.

Allow to specify timeout values using ENV vars (Tecnativa#2)

* Allow to specify timeout values using ENV vars
* Cleanup README formatting
* Sort env vars in alphabetical order
* Read config values straight from environment
* Revert code formatting

Signed-off-by: Andrejs Volkovs <[email protected]>