Background Compilation: configurable build task

[MahdiBM]

I don't always want to build to whole thing when background compilation is on.
We should be able to configure what build task to use when a background compilation is to be done.
Somewhat similar to how you'd choose a specific scheme in Xcode, when working on different things.

Per usual, I'll refer to vapor/penny-bot as an example. There are 9 different executables, and usually I work on one of the 2 bigger executables, GHHooksLambda or Penny, and sometimes building tests is not needed either.
One can always manually trigger a more complete build if needed when changing the configured background-compilation build task is not desired.