docs(guides): Make all demo context consistently

[Ivocin]

I read the guides and find that some demos aren't consistent.
This pr contains three parts:

• docs(guides): change var to let and move lodash under dependencies #2409 change var to let in index.js file
• docs(guides): change CleanWebpackPlugin arguments usage #2845 change CleanWebpackPlugin arguments usage in webpack.config.js file
• delete extra part in package.json to avoid changing the dependency vesion （now the webpack in package.json version is ^3.0.0)

[RedTn]

@Ivocin @EugeneHlushko What's status on this? Ran into bug with CleanWebpackPlugin on production guide and this PR resolves it. Need my help to take it over?

[EugeneHlushko]

@Ivocin can you resolve conflicts? an earlier pr to let const was merged

[Ivocin]

@Ivocin can you resolve conflicts? an earlier pr to let const was merged

Close, another pr has been merged.

[EugeneHlushko]

@Ivocin can you quickly open a new PR where you do changes for CleanWebpackPlugin from this pr's diff ?

[RedTn]

I can take it if he's unavailable

[EugeneHlushko]

Hi and thank you @RedTn but i already have a branch with such changes, just wanted to get original contributor to respond first

[EugeneHlushko]

Updated the branch and pushed the changes.