FR: allow a default watcherTask

[aspiers]

Currently you always have to specify a watcherTask argument to hardhat watch:

yarn hardhat watch <watcherTask>

It would be nice if a default key could be set in the watcher config to make this argument optional. In the (presumably common) case where only one watcherTask is configured, this would save having to type that argument, and it would also allow hardhat watch to make something sensible to happen without having to first check which watcherTasks are defined in the config.

[xanderdeseyn]

Good idea, I'll ship it asap

[aspiers]

Awesome thanks!