Add possibility to copy on start

[vladshcherbin]

It would be nice to have an option to write on start or on write. It's useful in watch mode when you don't want to copy files every time you bundle changes. The option can be named watch or smth.

[bengsfort]

Will tackle this today.

[bengsfort]

Sorry never got back to this. Due to the way the rollup plugin hooks work, this is actually quite difficult. The initial hook that happens on start does not provide enough information to know about the output directory, and quite a few of the plugin hooks everything works off of do not have access to the context methods such as adding a file/directory to be watched; so this actually requires quite a bit of work. I may take a look in the future, but I don't have time to dedicate to this feature request at the moment.