Coding style normalisation

- Added spacing around conditional statements and context differences
- Added an 'error' function so we can normalise error messages into stderr
- Wrapped all referenced variables in `{}`
- Tests now use double `[[]]` instead of `[]`
- `if`,`for` and `while` have `; then` on the same line
- If only one command is specified when we expect two, it will output a list of subcommands to include