Add an option to use ICU to locate word boundaries

Add a CMake option (USE_ICU) to enable the use of ICU to locate word
boundaries in strings.  If USE_ICU is enabled then split_text_node()
uses ICU's BreakIterator to split up a text node.  If USE_ICU is
disabled then split_text_node() uses the existing implementation to
split up a text node.

USE_ICU is disabled by default so this does not introduce a dependency
on ICU for existing litehtml users.