v1.88

Change-log:

• Make proper usage of cmake's package_config.
• Provide examples to consume the library.
• Apply proper renaming in accordance to imgui's backend changes.
• Inline backends include function in the same CMakeLists.txt.
• Set default CMAKE_INSTALL_PREFIX and allow for it to be overriden
  (closes #2).
• Change imgui branch from master to docking.
• Update readme.