Add rich text HTML support

I've used MTH for a while to help me to do some properly references of my blog articles. Recently I've found that *Rich Text HTML* format (from CoLT) is a pretty useful feature when combining with Evernote. It makes the whole references recording process even simpler and easier. However, CoLT only supports single page's URL copy so I do some hacks to MTH to add this feature in. It works quite flawlessly. But I think there should be some problem with private browsing mode since I cannot figure out what's going on at that part. Besides, sending HTML elements to clipboard needs another accessing method. Currently, it was just done through a simple protocol (message exchanging) inside the original function call flow. Maybe you can make this feature more native to MTH if you like it. (I hope so.)

## Changes

* .gitmodules

    For other users (not the author you) to check out through `git submodule` more easily

* Makefile
    1. `.PHONY` normalization
    2. Add 'clean` target to automatically remove packaged files and checksum

    P.S. Packaging seems to have some problem. Or it just lacks of support for developing on FreeBSD.

* content/multipletab/multipletab.js

    Rich text HTML format support