CORS support and minor cleanup

A merging of a few older PRs the most notable being the addition of CORS support with elements that support crossorigin tags.

Fix regression that permitted script tags to be injected

#111 revealed that we had experienced a regression of an earlier fix and that <script> tags could be injected by relying on the use of the uppercase Cyrillic i, as when Go lowercases this it would map the UTF-8 rune to an ASCII rune. The fix prevents this by retaining the ASCII escaping.

URL encoding fix

A minor release to fix a HREF sanitization bug that was resolved in #103

v1.0.3: Merge pull request #92 from theflyingcodr/element_regex

AllowElements regex support

Go mod support

Merge pull request #77 from SamWhited/support_modules

Support Go Modules

Initial semver for vgo and go modules.

Merge pull request #69 from Mungrel/patch-1

Add address to DefaultElementsWithoutAttrs