How can I change the resolution on my Mac to 720p (1280x720)? I'm looking to record some screencasts, and 720p (1280x720) seems to be the best choice for the resolution. It's supported by video sites like YouTube and Vimeo as one of the standard "HD" resolutions, but it's small enough that it can be comfortably viewed on smaller screens without needing to play the video back full screen.

However, there is no option for 1280x720 in my System Preferences. How can I get my Mac to use this resolution? I've seen SwitchResX and I've tried to make it work, but it's very complicated and I haven't had much luck with it. Are there any other utilities that do the same thing? Would buying a Mini DisplayPort -> HDMI adapter and hooking it up to a 720p TV work?
One quick tip for finding more resolutions is to hold the Option key while clicking Scaled in System Preferences > Displays. This definitely works in OS X El Capitan and I think it worked in Yosemite at least.

For example, without Option, my 1440x900 macbook air screen gives me 1440 x 900, 1280 x 800, 1152 x 720, and 1024 x 640. But when I add Option, it adds 1140 x 810, 1280 x 720 (the one you want), and 800 x 600.