Add --require-env to check-executables-have-shebangs

[chuckatkins]

Adds --require-env

• Require shebangs to invoke an interpreter through /usr/bin/env (e.g. #!/usr/bin/env python)
• Off by default

Adds --fix

• Rewrite existing shebangs to use /usr/bin/env
• Requires --require-env, since the hook cannot infer an interpreter when no shebang exists
• Off by default

Fixes #1273.