Add AllowURLSchemeWithCustomPolicy() to allow external fine grain control over allowed URLs.

Closes #6.
Fix typo "schems" -> "schemes" in docs for AllowURLSchemes.
Don't make urlPolicy type exported to keep public API bloat minimum.
Merge pull request #7 from shurcooL/feature/add-external-url-policy-ability

Add AllowURLSchemeWithCustomPolicy() to allow external fine grain control over allowed URLs.