Allow config customization

[desmondinho]

At the moment there is no way to configure and extend TwMergeConfig as mentioned in the first pre-release of this project.
This limitation becomes annoying when you want to extend the default TailwindCSS config in your ASP.NET Core web projects but merging results in unexpected behavior.

Proposed Changes:

Allow providing customTwMergeConfig with the following abilities:

• Provide a custom cache size
• Provide a custom prefix
• Provide a custom separator
• Extend class groups