Add instructions to install include dir and headers

Author: SolarLiner

include/meson.build

@@ -1 +1,9 @@
 sass_inc = include_directories('.')
+version_h = configure_file(input: 'sass/version.h.in', output: 'version.h',
+  configuration: {'PACKAGE_VERSION': meson.version()},
+  install: true,
+  install_dir: get_option('includedir') / 'sass'
+)
+
+install_subdir('sass', install_dir: get_option('includedir'), exclude_files: ['meson.build', 'sass/version.h', 'sass/version.h.in'])
+install_headers('sass.h', 'sass2scss.h')