pub fn string_to_platform_spec(s: Option<&str>) -> Result<PlatformSpec> {
    match s {
        Some("current") => Ok(PlatformSpec::current()?),
        Some("always") => Ok(PlatformSpec::Always),
        Some("any") => Ok(PlatformSpec::Any),
        Some(triple) => Ok(Platform::new(triple.to_owned(), TargetFeatures::Unknown)?.into()),
        None => Ok(PlatformSpec::Any),
    }
}
Parse a given triple, the string "current", or "any", into a platform.

TODO: This should eventually support JSON specs as well, probably.
guppy-cmdlib/src/lib.rs