Support using environment variables as input data

This uses the $_ENV superglobal, which is an empty array unless the
variables_order ini directive contains "E"; see
http://php.net/manual/en/ini.core.php#ini.variables-order.

Environment variables have lower precedence than input data passed via
other means, e.g.

    $ echo '{{USER}}' | twigc --env
    peets
    $ echo '{{USER}}' | twigc --env -p USER=foo
    foo

README.md

@@ -19,6 +19,7 @@ Options:
   -V, --version        Display version information
       --credits        Display dependency credits (including Twig version)
   -d, --dir=DIR        Add search directory to loader (multiple values allowed)
+      --env            Treat environment variables as input data
   -e, --escape=ESCAPE  Set autoescape environment option
   -j, --json=JSON      Pass variables as JSON (dictionary string or file path)
   -p, --pair=PAIR      Pass variable as key=value pair (multiple values allowed)

src/Twigc/DefaultCommand.php

@@ -63,6 +63,12 @@ class DefaultCommand extends Command {
 				InputOption::VALUE_REQUIRED | InputOption::VALUE_IS_ARRAY,
 				'Add search directory to loader'
 			)
+			->addOption(
+				'env',
+				null,
+				InputOption::VALUE_NONE,
+				'Treat environment variables as input data'
+			)
 			->addOption(
 				'escape',
 				'e',
@@ -349,6 +355,10 @@ class DefaultCommand extends Command {
 			}
 		}
 
+		if ( $input->getOption('env') ) {
+			$inputData = array_merge($_ENV, $inputData);
+		}
+
 		// Validate key names now
 		foreach ( $inputData as $key => $value ) {
 			if ( ! preg_match('#^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$#', $key) ) {