Support dynamic library. (#225)

- Let the global `BUILD_SHARED_LIBS` dictates whether the library should
  be built statically or dynamically. The cmake's default is statically.
- Add library version and symlink.

This lead to the following install tree.
.
├── include
│   └── ftxui
│       ├── component [...]
│       ├── dom [...]
│       ├── screen [...]
│       └── util [...]
└── lib
    ├── cmake
    │   └── ftxui
    │       ├── ftxui-config.cmake
    │       ├── ftxui-config-version.cmake
    │       └── ftxui-config-version-noconfig.cmake
    ├── ftxui-component.so -> ftxui-component.so.0.10.369
    ├── ftxui-component.so.0.10.369
    ├── ftxui-dom.so -> ftxui-dom.so.0.10.369
    ├── ftxui-dom.so.0.10.369
    ├── ftxui-screen.so -> ftxui-screen.so.0.10.369
    └── ftxui-screen.so.0.10.369

Fixed: https://github.com/ArthurSonzogni/FTXUI/issues/223