Refactor CMake binding configuration

### What is the desired addition or change?



This comes out of the discussion for the R bindings: https://github.com/mlpack/mlpack/pull/2556#discussion_r470272755



At the end of `src/mlpack/CMakeLists.txt`, there is a section for each binding type.  It would be cleaner if we instead created macros like `post_markdown_setup()` for each of the existing binding languages (Julia, Go, Python) in each binding directory's `CMakeLists.txt` (i.e. `src/mlpack/bindings/*/CMakeLists.txt`), and then simply called those macros.



### What is the motivation for this feature?



A cleaner CMake configuration. :)



### If applicable, describe how this feature would be implemented.



So, in an ideal world, there would be a block at the end of `src/mlpack/CMakeLists.txt` like this:



```

post_python_setup()

post_julia_setup()

post_go_setup()

post_markdown_setup()

```



and that would be the only code in that file related to the bindings.



### Additional information?



This is a pretty simple refactoring, so it could be a nice starter task for anyone looking to get involved with mlpack.  Some CMake knowledge is probably necessary, but CMake isn't too hard to learn. :)

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