Add postMessage to Programming Interface

[zzjin]

When we want to use programming interface to auto run some command for user, we encountered issues when main site and ttyd site are host under subdomains.

for example:
we run a web site that helps show some useful debug commands, under a.com, with one ttyd iframe using sub domain: ttyd.a.com.
when one clicks debug command at a.com's page, we want to auto pate and run inside ttyd.a.com's iframe, but there shown errors:

Blocked a frame with origin "https://a.com" from accessing a cross-origin frame.
    at <anonymous>:1:68
(anonymous) @ VM202:1

So wish there have some postmessage programming interface to communicate to ttyd pages to just run some limit commands like paste&run command.

[tsl0922]

I'm OK to merge a PR for this feature, If you know how to implement it.