some minor changes in Jekyll options and CSS:

1. ignore by-products from Jekyll;
2. set the baseurl (for Github), and use shorter permanent links;
3. do not show the horizontal scroll bar for <pre> when not necessary;
4. add a horizontal line above the footnotes section;
add a build script
add an R Markdown post under _source, and build the website
we can also use RStudio and a Makefile to build the website by clicking a button
add a readme