Download and install main.min.js in BlockLauncher.
You can enter commands, or Javascript to run by prepending javascript: or js:, ie js: ModPE.showTipMessage("Hello world!");
This mod also features Minecraft 1.9's command block easter eggs.
Using Javascript is great, as you don't need to rely on having other mods installed, however you do need some knowledge of the ModPE functions. You can refer to the ModPE API here.
javascript: var d = new Date(); ModPE.takeScreenshot(d.now());
If the command block has toggle mode enabled, it will run when it gets powered but also when it gets unpowered.
Please use the official link when distributing. That way, the link will always be up to date with the latest version.
It would also be great if you could link back to this github repo or the MCPE Stuffs article :)