Disable playground button on listings in ch 12 that use CLI args

You can't pass CLI args to the playground, so it doesn't make sense to
run them in the browser. Also, these were marked as should_panic,
because if you *do* run them in the browser or without arguments, they
panic, but that's not the happy path we're writing, so "should" panic
isn't really correct.

Fixes rust-lang#2370.

Author: carols10cents

src/ch12-01-accepting-command-line-arguments.md

@@ -102,7 +102,7 @@ throughout the rest of the program. We do that in Listing 12-2.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,should_panic
+```rust,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-02/src/main.rs}}
 ```
 

src/ch12-02-reading-a-file.md

@@ -22,7 +22,7 @@ shown in Listing 12-4.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,should_panic
+```rust,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-04/src/main.rs:here}}
 ```
 

src/ch12-03-improving-error-handling-and-modularity.md

@@ -119,7 +119,7 @@ Listing 12-6 shows the improvements to the `parse_config` function.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,should_panic
+```rust,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-06/src/main.rs:here}}
 ```
 
@@ -187,7 +187,7 @@ shows the changes we need to make.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,should_panic
+```rust,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-07/src/main.rs:here}}
 ```