Export fails if html has no <base>

[wyozi]

Describe the bug
Export will fail with a bad error message if Svelte has a route serving .html that does not contain <base> tag.

The problematic code is

sapper/src/api/export.ts

Lines 188 to 190 in 4a0ffd1

	const base_match = /<base ([\s\S]+?)>/m.exec(cleaned);
	const base_href = base_match && get_href(base_match[1]);
	const base = resolve(url.href, base_href);

If there is no base_match, there won't be base_href either so resolve will receive undefined as the second argument.

I think it might be safe to replace base_href with basepath received from options, but not sure.

[Conduitry]

I believe this is the second of the two issues that #1118 is intended to address.