Support flexible build directory naming & structure for enabling multiple build types

[krishnakumarg1984]

Currently, the build directory name needs to be hard coded in the configuration. It would be helpful to have a more flexible approach wherein a build_dir_prefix strings is provided to be user configurable. This will enable C++ developers to have access to multiple type of builds.

Examples:

• build_dir_prefix: cmake_build_
  In the above example, the build directory will become cmake_build_Debug, cmake_build_Release, wherein the appropriate suffix is chosen by the build type

• build_dir_prefix: build/
  In this above example with the trailing /, a classical nested hierarchical structure of various types of builds can be facilitated

build
├── Debug
├── MinRelease
├── Release
└── RelwithDebInfo

The above is a classical build tree for multiple type of builds, and is easily done through cmake invocations at the command-line, but this is not currently possible with this plugin.
The vimscript plugin, [cmake4vim](https://github.com/ilyachur/cmake4vim) already supports this, and I am using it now in my projects, but I'd like to move to this lua-based plugin, mainly for its integration with nvim-dap.

Of course, to prevent breaking changes to existing users' configurations, the build_dir_prefix behaviour should be activated only when cmake_build_directory is set explicitly to an empty string. Otherwise, we simply ignore the build_dir_prefix setting, and simply overwrite builds each time to the folder configured in cmake_build_directory (the current behaviour).

[Civitasv]

Thanks for your suggestion. I will implement this option soon.

[Civitasv]

Hi, please see commit f726e66. I've implemented this feature.

require("cmake-tools").setup({
...
  cmake_build_directory = "",
  cmake_build_directory_prefix = "build/",
...
})