Minor updates and improvements

Hi all,

this is a minor release which adds some useful functionality, improves the documentation, and fixes minor bugs.

New functionality

• more of the front page layout can now be controlled from the YAML header in index.Rmd (font size of the title, size of whitespace before and after the university logo)
• overflowing lines in code blocks are now wrapped
• table of contents can now be linked just by the page number, instead of the entire heading, which is preferable when links are colored (thanks @shirdekel!)

Minor changes & improvements

• example bibliographies now live in a folder instead of the root directory
• improved documentation: the 'how to use' chapter now outlines the structure of oxforddown

Bug fixes

• abstract is only shown if provided (thanks @kasnder!)