Implement packager and installer hooks

Implement packager and installer hooks

==Packager hook
If not --pecl nor --git are explicitely given, look for a
pharext_install.php in --source. This script will be exectuted by the
Packager. It must return a callable with the following signature:

function(Packager $pkg, $path) : function(Packager $pkg, $path);

So, the callback should return another callable.
The primary callback is meant to set things like --name and --release,
so you don't have to on the command line; build automation FTW.
The secondary callback is meant to create the file iterator of the
source dir, but if you're in a git clone, you might easily just return a
new pharext\GitSourceDir and be done.

==Installer hook
The packager will look for a pharext_install.php file within the root of
the source dir. This script will be executed by the Installer; it must
return a callable with the following signature:

function(Installer $installer) : function(Installer $installer);

So, again, the callback should return another callable.
The primary callback is meant to add your own command line arguments to
the CliArgs parser, and the secnodary callback is meant to proccess
those args.

For --pecl source dirs a pharext_install.php script is automatically
generated from the package.xml.

==Examples for pecl_http

pharext_package.php
---8<---
<?php

namespace pharext;

return function(Packager $packager, $path) {
	$args = $packager->getArgs();
	$args->name = "pecl_http";
	$args->release = current(preg_filter("/^.*PHP_PECL_HTTP_VERSION\s+\"(.*)\".*$/s", "\$1", file("../http.git/php_http.h")));
	return function (Packager $packager, $path) {
		return new GitSourceDir($packager, $path);
	};
};
?>
--->8---

pharext_install.php
---8<---
<?php

namespace pharext;

return function(Installer $installer) {
	$installer->getArgs()->compile([
		[null, "with-http-zlib-dir", "Where to find zlib",
			CliArgs::OPTARG],
		[null, "with-http-libcurl-dir", "Where to find libcurl",
			CliArgs::OPTARG],
		[null, "with-http-libevent-dir", "Where to find libev{,ent{,2}}",
			CliArgs::OPTARG],
		[null, "with-http-libidn-dir", "Where to find libidn",
			CliArgs::OPTARG],
	]);

	return function(Installer $installer) {
		$opts = [
			"with-http-zlib-dir",
			"with-http-libcurl-dir",
			"with-http-libevent-dir",
			"with-http-libidn-dir",
		];
		$args = $installer->getArgs();
		foreach ($opts as $opt) {
			if (isset($args[$opt])) {
				$args->configure = "--$opt=".$args[$opt];
			}
		}
	};
};
?>
--->8---