Code ligatures

Roboto Mono is a font widely-used by people to use in code editors, myself included. Some newer mono fonts for use in code editors contain the addition of ligatures for common programming multi-character combinations, such as [Fira Code](https://github.com/tonsky/FiraCode) and [JetBrains Mono](https://www.jetbrains.com/lp/mono/).

Roboto Mono lacks these ligatures, and as such people have made scripts to merge the ligatures of these fonts into Roboto Mono. However, as they come from a different font altogether, they are quite obviously not in accordance to the style of Roboto Mono. It would be great if official code ligatures could be added.