Added html support using the latest prettier 1.15 version

[premithk]

Before submitting a pull request, please make sure the following is done...

1. Fork the repo and create your branch from master.
2. If you've added code that should be tested, do a sanity check on vim8 and neovim
3. If you've changed APIs, update the README and documentation ./doc/prettier.txt

Please use the simple form below as a guideline for describing your pull request.

Thanks for contributing to vim-prettier

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Summary
Upgraded the prettier version
Added support for html files

Test Plan
Tested using a plain html file
[...]

[mitermayer]

Thank you !