add json emit mode

[calebcartwright]

Closes #1193 and refs #3274 (the --message-format options are json, human, and short)

Since each file is formatted and emitted individually, I had to keep track of the number of files with mismatches that had been emitted within the JsonEmitter in order to ensure that the resulting output would be a valid JSON string. This required updating emit_formatted_file in Emitter to take a mutable reference.

As an example for what the emitted JSON looks like, below is a human-friendly version (spacing adding) of the json output emitted for this file:

[  
  {  
    "name":"tests/writemode/source/json.rs",
    "mismatches":[  
      {  
        "original_begin_line":5,
        "original_end_line":7,
        "expected_begin_line":5,
        "expected_end_line":5,
        "original":"fn foo_expr() {\n    1\n}",
        "expected":"fn foo_expr() { 1 }"
      },
      {  
        "original_begin_line":9,
        "original_end_line":11,
        "expected_begin_line":7,
        "expected_end_line":7,
        "original":"fn foo_stmt() {\n    foo();\n}",
        "expected":"fn foo_stmt() { foo(); }"
      },
      {  
        "original_begin_line":13,
        "original_end_line":15,
        "expected_begin_line":9,
        "expected_end_line":9,
        "original":"fn foo_decl_local()  {\n    let z = 5;\n   }",
        "expected":"fn foo_decl_local() { let z = 5; }"
      },
      {  
        "original_begin_line":17,
        "original_end_line":19,
        "expected_begin_line":11,
        "expected_end_line":11,
        "original":"fn    foo_decl_item(x: &mut i32) {\n    x = 3;\n}",
        "expected":"fn foo_decl_item(x: &mut i32) { x = 3; }"
      },
      {  
        "original_begin_line":21,
        "original_end_line":21,
        "expected_begin_line":13,
        "expected_end_line":13,
        "original":"   fn empty()     {",
        "expected":"fn empty() {}"
      },
      {  
        "original_begin_line":23,
        "original_end_line":23,
        "expected_begin_line":15,
        "expected_end_line":15,
        "original":"}",
        "expected":"fn foo_return() -> String { \"yay\" }"
      },
      {  
        "original_begin_line":25,
        "original_end_line":29,
        "expected_begin_line":17,
        "expected_end_line":20,
        "original":"fn foo_return() -> String {\n    \"yay\"\n}\n\nfn foo_where() -> T where T: Sync {",
        "expected":"fn foo_where() -> T\nwhere\n    T: Sync,\n{"
      },
      {  
        "original_begin_line":64,
        "original_end_line":66,
        "expected_begin_line":55,
        "expected_end_line":55,
        "original":"fn lots_of_space                      ()                                                           {\n                           1                 \n}",
        "expected":"fn lots_of_space() { 1 }"
      },
      {  
        "original_begin_line":71,
        "original_end_line":72,
        "expected_begin_line":60,
        "expected_end_line":60,
        "original":"    fn dummy(&self) {\n    }",
        "expected":"    fn dummy(&self) {}"
      },
      {  
        "original_begin_line":75,
        "original_end_line":75,
        "expected_begin_line":63,
        "expected_end_line":64,
        "original":"trait CoolerTypes { fn dummy(&self) { ",
        "expected":"trait CoolerTypes {\n    fn dummy(&self) {}"
      },
      {  
        "original_begin_line":77,
        "original_end_line":77,
        "expected_begin_line":66,
        "expected_end_line":66,
        "original":"}",
        "expected":""
      },
      {  
        "original_begin_line":79,
        "original_end_line":79,
        "expected_begin_line":67,
        "expected_end_line":70,
        "original":"fn Foo<T>() where T: Bar {",
        "expected":"fn Foo<T>()\nwhere\n    T: Bar,\n{"
      }
    ]
  }
]

[topecongiro]

LGTM, thank you!