Allow PRs to be tried out on run.dlang.io

[Quirin Schroll]

It might be worth adding the option to run code through a DMD 
with proposed changes in an open PR on the run.dlang.io site. 
Below `dmd-nightly` add a `dmd-pr`, which when selected, adds 
another input before command-line args to enter a DMD PR number. 
That should work if the PR passes all checks. (An alternative to 
adding a new input would be to interpret the first command-line 
argument as the PR number.)

This would allow the community to quickly try out changes that 
are not even in the compiler yet.