Respect user-supplied quoting (#8)
Currently it's impossible to have 'lazily' evaluated environment
variables like
```
FOO='${BAR}'
```
where `${BAR}` is evaluated at runtime.
This is because we strip any quotes (single or double) and replace them
with double quotes, and then `${BAR}` is expanded.
Instead, we can check if the value is already quoted, and only supply
our own double quotes if it's not.