Make <base> optional

[thgh]

Adds an option baseTag: false to sapper.middleware to opt out of the <base> tag

Resolves #904 => issues with svg and css url()
Base tag was introduced in #180

Could perhaps be extended to support #866 (I have not checked effect on sapper export)
Could also be extended to support #600 (By adding a baseOrigin option)

[benmccann]

@thgh I think this PR will end up being blocked until we figure out how to handle configuration in Sapper. I raised an RFC since I'm also blocked by that decision. Please take a look and feel free to leave any feedback there

[benmccann]

It was reported in #1237 that export won't work if base is not present. I haven't tested myself, but assuming that's true then I imagine this will break export

[thgh]

Looks like export only supports base indeed, that should be easily fixed by using this url as base.

[benmccann]

#1208 is trying to solve the export problem, so I think if it gets merged first then it will be fine

[benmccann]

Thanks for putting together this PR! Sapper never had a good method of handling configuration, which made it difficult to know how to integrate this feature. However, that's been addressed in SvelteKit which has a svelte.config.cjs file. I'm going to go ahead and close this PR since we won't be adding the feature to Sapper, but if you'd like to check out SvelteKit we will be reviewing PRs for features there