Update Meson build system

The following features from the CMake build have been implemented:
 * Library version retrieved from unity.h.
 * Extension support.
 * Library, header, and package configuration file installation.

This commit is entirely based on existing work by Owen Torres.

src/meson.build

@@ -4,10 +4,12 @@
 #
 # license: MIT
 #
-unity_dir = include_directories('.')
 
-unity_lib = static_library(meson.project_name(),
-    'unity.c',
-    include_directories: unity_dir,
-    native: true
+unity_inc += include_directories('.')
+unity_src += files('unity.c')
+
+install_headers(
+  'unity.h',
+  'unity_internals.h',
+  subdir: meson.project_name()
 )