Add support for defining input/output files for CustomConfig

[hbragge]

Currently there is no way to define input files or output artifacts in CustomConfig, thus there is no way for bake to notice if input files have changes and prevent unnecessary runs. Commands something like below could be added to introduce this.

Probably also a new keyword for the actual steps is needed (I used "Commands" in the example), because normally PreSteps/PostSteps are run regardless if input files have changed or not.

CustomConfig generate {
Set MyScript, value: "myscript.rb"
Set InputFile1, value: "input1.txt"
Set InputFile2, value: "input2.txt"
Set OutputFile1, value: "output1.h"
Set OutputFile2, value: "output2.h"
InputFiles "$(MyScript)"
InputFiles "$(InputFile1)"
InputFiles "$(InputFile2)"
OutputFiles "$(OutputFile1)"
OutputFiles "$(OutputFile2)"
Commands {
CommandLine "$(MyScript) -i $(InputFile1),$(InputFile2) -o $(OutputFile1),$(OutputFile2)"
}
}