Add webpack compilation object to processfn

[samsaggace]

Summary

Enables to manipulate the webpack build to add an asset file to the build. Discussed in https://github.com/slackhq/csp-html-webpack-plugin/issues/52

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've written tests to cover the new code and functionality included in this PR.
• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).

[CLAassistant]

All committers have signed the CLA.

[samsaggace]

Regarding tests, I added the argument to the list of expected arguments, but not that it's really the webpack compilation object, I don't know if it's possible or needed

[codecov[bot]]

Codecov Report

Merging #81 (f55370b) into master (96491f9) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #81   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          108       108           
  Branches        19        19           
=========================================
  Hits           108       108           

Impacted Files	Coverage Δ
plugin.js	100.00% <100.00%> (ø)

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[samsaggace]

Repushed : reinstall webpack-sources dependency with node 10 and npm 6 installed to avoid big diff in package-lock.json

[samsaggace]

Hi @AnujRNair,

I think I have implemented your suggestions but please tell me if there is something else I need to do before this PR could be merged.

Thanks

[AnujRNair]

Once I have had a second to confirm whether a regression was released or not in 5.0.1 as reported in https://github.com/slackhq/csp-html-webpack-plugin/issues/80, I will also release this change to NPM.

Thanks for the contribution!