Add support for custom initialisms

[zx8]

In my codebase, I use initialisms that are domain-specific and not included in Go's common initialisms.

The underlying strcase package supports customization, so I think it should be fairly simple to implement in tagliatelle: https://github.com/ettle/strcase#customization

If implemented, it would be great to also raise a PR to support the new configuration in golangci-lint.

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Example of the problem:

json(goCamel): got 'docURLs' want 'docUrLs'

[zx8]

Closing as this is 2.5 years old without any movement.