Consider adding an option to "option()" for overriding the env var name

Currently, if you want to read an option from a different env var than TS would look for, you'd have to do the following:

@ts.settings
class Settings:
    ssh_key_file: Path = Path(os.getenv("SSH_PRIVATE_KEY_FILE", ""))

Maybe it would be a good idea to add something like this:

@ts.settings:
class Settings:
    ssh_key_file: Path = ts.option(default="", env_var="SSH_PRIVATE_KEY_FILE")

The problem with this is that it adds configuration for loaders to TS options that do not know which loaders will actually be used.