Enable optional features for Perl code chunks

[kiwiroy]

This PR aims to enable optional features when running Perl code chunks.

From perlrun

  -E commandline
            behaves just like -e, except that it implicitly enables all optional features (in the main compilation unit). See feature.

Current behaviour

An attempt to use say, as in say "Hello, World!";, is met with the following error.

Error in engine(options) : 
  String found where operator expected at -e line 2, near "say "Hello, $user""
	(Do you need to predeclare say?)
syntax error at -e line 2, near "say "Hello, $user""
Execution of -e aborted due to compilation errors.
Calls: <Anonymous> ... process_group.block -> call_block -> block_exec -> in_dir -> engine

[yihui]

Thanks! I don't know much about Perl, so I'll just trust you.

BTW, could you add an item in NEWS.md to mention this change?

[kiwiroy]

Awesome. NEWS.md updated. Thanks.

[yihui]

Perfect. Thanks!