Add keywords macro

[tecosaur]

At the moment keywords are made obvious via redefining \emph to use italic in a different font weight, and \textit used where \emph would have been previously. However, this goes against the design philosophy of LaTeX and so it would be worth leaving \emph as is and adding a \keyword / \kwd / etc. command for this purpose.

Todo:

• Add \keyword (whatever the name may be) command
• Add styling option, probably in some separate macro that can be redefined like \keywordstyle
• Add option that puts keywords in a margin (if using margins) — see https://tex.stackexchange.com/questions/257797/how-to-add-a-key-word-beside-a-paragraph