Add configuration features to README
Adding support for preamble text in config file

This allows to add text to the latex preamble among multiple files,
e.g., adapting fonts for multiple markdown files.
Add scoped attribute to <style>

With HTML5, the scoped attribute for <style> has been introduced, and it
is mandatory for style definitions that are not in <head>.
Create unique IDs for the <img> tags
Merge pull request #13 from shuber2/master

Adjust readme, add preamble option for config file