Implement support for arbitrary try builds

[Kobzol]

With rust-lang/rust#124631, the rust CI will be able to run an arbitrary CI job in a try build. It would be great to add support for this in bors.

Arbitrary try build CI jobs are parsed from a special line with the format try-job: <job-name>, which should be in the commit message of the latest commit on the PR. Currently, bors sets the Auto merge commit message to the PR description, so users have to modify it. With new bors, it should instead append these special lines directly to the merge commit message.

Steps:

1. Extend the parsing of a @bors try command so that it will be able to receive the names of arbitrary CI jobs.
   Something like @bors try jobs=<name1>,<name2>,...
2. For each job name in the command, output a single line with the contents try-job: <job-name> to the commit message of the try build

And that's pretty much it. We could also implement some sanity checks, e.g. if the user specifies 50 jobs, bors should error out (rust-lang/rust#124631 currently allows specifying 10 jobs max.).