Need to expose start and eng tag names

[yukinying]

Currently the start tag name and end tag name are stored in this.tags[0] and this.tags[1] respectively, and the upstream project html-purify is using these internal variable directly. Unfortunately, this.tags, even may have stored value, would be valid for use only in certain HTML state.

We should construct the functions getStartTagName and getEndTagName that would only return tag name when they are in the correct HTML state.

I will push a few commits to make this happen.

@maditya @neraliu @adon-at-work