Support generic commands

[divmain]

Support a commands option in Interlock config files. This option would accept an object, with command strings for keys and functions for values. Each command can be invoked by ilk run COMMAND_KEY_STRING and fully supports the presets composability.

Each command function would be provided an object for its single argument, with several keys. Among them:

• compile, a function that accepts a Interlock config and returns a promise
• config, the validated config object
• exec, a promise interface to invoke external processes
• run, a function that accepts a single argument (another command key string) and returns a promise

The function should return a Promise. Commands can be composed (across presets) by using Promise.all (for parallel) or Promise-chaining (for sequence).