added an option to specify 'only' as a selector for files to purge

[somebody32]

Proposed changes

Added an option to have more fine-grained control over lists of assets purge runs.

Example:

new PurgecssPlugin({
  paths: glob.sync(`${PATHS.src}/*`),
  styleExtensions: ['.css']
  only: 'my_huge_css_to_purge'
  // or as an array: 
  // only: ['my_huge_css_to_purge', 'and_another_one']
})

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Further comments

I will change docs later if you think it is ok to have that option

[Ffloriel]

Thanks for the contribution.
I think it is ok to have the option. Can you provide an example in the examples folder to confirm the plugin works as expected? You can base yourself on the simple one.

[somebody32]

@Ffloriel done!

[Ffloriel]

Thanks for the PR