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.

auto/extract_version.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+import re
+import sys
+
+ver_re = re.compile(r"^#define\s+UNITY_VERSION_(?:MAJOR|MINOR|BUILD)\s+(\d+)$")
+version = []
+
+with open(sys.argv[1], "r") as f:
+    for line in f:
+        m = ver_re.match(line)
+        if m:
+            version.append(m.group(1))
+
+print(".".join(version))
+