Make all paths supplied by flags absolute

[SarahFrench]

Closes #34

This PR makes the --goldens and --tests flag values be converted to absolute paths before they're used. This avoids the problem described in the linked issue.

I also changed how the path to the Terraform binary is made absolute, so that it's consistent. The function filepath.Abs is implemented pretty much the same way as the code that's being replaced:

// src/path/filepath/path.go
func Abs(path string) (string, error) {
	return abs(path)
}

// src/path/filepath/path_unix.go
func abs(path string) (string, error) {
	return unixAbs(path)
}

// src/path/filepath/path.go
func unixAbs(path string) (string, error) {
	if IsAbs(path) {
		return Clean(path), nil
	}
	wd, err := os.Getwd()
	if err != nil {
		return "", err
	}
	return Join(wd, path), nil
}

---

I'm unsure of any pitfalls around converting paths to absolute paths, which is the main thing I'd like to address during review. E.g. usage on non-unix machines?

[liamcervante]

Looks good! I think filepath is actually architecture specific anyway, so this should work across windows/linux/mac/etc.