Add flag for serving SPAs

[ro-savage]

Would be great to a flag for SPAs that adds two features

• Changes caching options so that index.html has a cache of 0, and all other files have the amount passed in the --cache flag (and maybe a sensible default if not cache flag was passed)
• Changes the folder redirect rule so that all files that aren't found get redirected back to /index.html

Happy to make PR for this.

[ro-savage]

Looks like you already have the option to always save index under -single.

So how about when using this flag, we add better caching defaults?

[leo]

Moving this discussion to the PR