Add comment styles; don't choke on `#!/usr/bin/env foo=bar`...

- Add `--` and `"` style single-line comments.
  - Rearrange `elsif`s so `"` comments don't interfere with removing double-quoted strings.
- Don't fail to detect valid `#!/usr/bin/env` shebangs which actually set environmental variables.
Do this first so Vim comments can be stripped.