Make the beginning of a new build more explicit in watch mode

[ghost]

Print the following on every new build in watch mode:

                                          
********** NEW BUILD **********

[rgrinberg]

Rather than this change, what do you think about making the new setting for watch the default? I think it's much better to just see the current status rather than what came before it.

Also, perhaps we should make sure that this addresses #1970 as well.

[ghost]

I haven't tried the new setting much, but last time I tried it I found something annoying: if you have multiple errors, they are all cleared as soon as a new build starts, which is annoying.

In any case, this change seems good to me even if we change the default. What do you think?

[rgrinberg]

Sure, sounds good to me. What about some colors btw? :)

@bobzhang would be it be possible to integrate this with vs code?

[ghost]

It's bold and in bright green :)