Adding option to allow double quotes instead of single quotes in HTML tags (issue #1)

As mentioned by @faceleg in [issue #1](https://github.com/tristen/vim-sparkup/issues/1), it would be nice to have an option for double vs. single quotes for HTML attributes. My company's standard is double, so I have to use that.

These commits add a command-line option called `--double-quote` to the python script. If the option is present, the script renders HTML attributes with double quoted values. I've also added an option called `g:sparkupDoubleQuote` to the vim script. If set to true, that option will cause vim to pass `--double-quote` to the python script, and thus switch it to use double quotes. Simply add `let g:sparkupDoubleQuote = 1` to your .vimrc or other configuration file.