YCMEPHelper: Do not fail bootstrap if CMAKE_DISABLE_FIND_PACKAGE_YCM is enabled
YCMBootstrap: Handle USE_SYSTEM_YCM flag

If this flag is explicitly set to false, we just skip to bootstrap.
AppVeyor: Print cmake version
YCMInternal: Use string(REPLACE) instead of string(REGEX REPLACE)
YCMInternal: On Windows replace slashes in filenames with escaped backslashes
YCMInternal: Escape brackets in filenames
Do not create update-sha1sums target on windows
Generate documentation