Split gubmo and litehtml to different CMake projects with different
compile languages.
Move 'src/strings.h' to 'src/gumbo/visualc/include/strings.h'.
Remove '../src/' in '#include' directive in litehtml.h and
add target_include_directories() in CMake project.
Set C/C++ standard flags by set_target_properties() in CMake project.
Do not set build type and C/C++ compiler flags in library project.
Half handling of CSS pseudo-elements (started with ::) - currently just removed from processing.
Added document_container::create_element - here you can create own implementation for elements.
litehtml::element is now lightweight. litehtml::el_text is inherited from litehtml::element. The old code with pre-parsed styles implementation was moved into litehtml::html_tag class.
