Your desire has already been fulfilled.  See [SOX](http://stackapps.com/questions/6091/stack-overflow-extras-sox).  It adds 50-some improvements to the Stack Exchange UI.  This includes a number of enhancements to the post editor, one of which is automated keycap markup tags (select text, click button, <kbd>instant_keycap_formatting</kbd>).

The list of SOX features is [here](https://github.com/soscripted/sox/wiki/Features).  Installation instructions and the install link are contained in that first Stack Apps link, first line of this answer.