Add watch and cleanOnApply options

[OJezu]

Has tests, by default is backward compatible. Fixes #29. Alternative to #32.

TODO:
When watching, keep list of files generated in previous compilation, and upon "emit" or "done" remove those, which are not present in current compilation.

[coveralls]

Coverage increased (+0.5%) to 94.286% when pulling 70b808e on OJezu:feature/add-watch-and-cleanOnApply-options into 4ae2022 on johnagan:master.

[nassimbenkirane]

Hi !
I'm looking for a similar functionality for my project. Unfortunately it doesn't work as expected when you have multiple entry points in your webpack config. A partial update on the watched files, will delete the other files ...

Example, if I have

dist/foo.bundle.js
dist/bar.bundle.js

If i change a file that is in foo, but not in bar, using {watch: true}, I will only have

dist/foo.bundle.js

and bar.bundle.js will have disappeared

[OJezu]

@nassimbenkirane

Does this also happen without {watch: true} or does it work as expected then?
As I said in TODO, full implementation should track which file were built previously and delete only them.

[aikar]

After reviewing both of these PR's, I decided to go another route and make my own plugin for now:
https://github.com/aikar/webpack-auto-clean-build-plugin

But I think this plugin should be modified to support this style of cleaning my plugin is providing AFTER the initial compile.

It should:
on webpack, pre first compile: wipe full
on incremental compile (webpack watch mode), remove abandoned chunks as my plugin is doing.

If this plugin is updated to support this (to not wipe entire build folder every compile), then i wouldn't need mine.

I don't think this plugin needs new config options for watch/apply. It should just do it.

[johnagan]

Sorry for the delay @OJezu - I'm happy to bring this in if we can resolve the conflicts.

[OJezu]

I need first to do some additional testing, I was using this a bit, and I noticed some possible issues.

[johnagan]

Closing in favor of #32