There are problems associated with `InterpreterOptions` being a string,
chiefly that it isn't safe to assume where individual Python interpreter
command line arguments begin and end simply by splitting the string on
space characters. This isn't so much of a concern while the value of
`InterpreterOptions` is just appended to the main .pex shebang in
please_pex, but it'll become more of a concern when we instead rely on a
native-code .pex preamble to invoke the interpreter because we'll be
passing arguments variadically to `execvp(3)`.

Make `InterpreterOptions` a repeatable configuration option and pass
each value to please_pex as a separate `--interpreter_options` command
line option (which itself also becomes repeatable). This isn't a
breaking change because the values passed via `--interpreter_options`
are for now ultimately just being stringified via a space-delimited join
and appended to the .pex shebang, but the distinction will become
meaningful when we implement a native-code .pex preamble.